Radio transmission apparatus and radio transmission method

ABSTRACT

When transmission is performed using the OFDM-CDMA method, subcarriers # 1  through #m, # 2   m +1 through # 3   m   , #3   m +1 through # 4   m , and # 4   m +1 through # 5   m  in which spread signals are allocated only in the frequency axis direction, and subcarriers #m+1 through # 2   m  in which spread signals are allocated in both the frequency axis direction and the time axis direction, are formed, and these are transmitted simultaneously.

TECHNICAL FIELD

The present invention relates to a radio transmitting apparatus andradio transmission method for use in a radio communication system thatemploys CDMA (Code Division Multiple Access) and OFDM (OrthogonalFrequency Division Multiplexing) technologies.

BACKGROUND ART

Conventionally, in a radio communication system combining OFDM and CDMA(hereinafter referred to as “OFDM-CDMA”), the combination of thecharacteristic of resistance to frequency selective fading that is anadvantage of OFDM modulation, and the characteristic of excellentinterference resistance through spreading gain that is an advantage ofCDMA, results in an ability to implement high-speed, high-qualitycommunications.

OFDM-CDMA methods broadly comprise a time domain spreading method and afrequency domain spreading method. With the time domain spreadingmethod, spread data that have been spread on a chip-by-chip basis bymeans of a spreading code are arranged in the time direction within thesame subcarrier. With the frequency domain spreading method, on theother hand, spread data that have been spread on a chip-by-chip basisare assigned to different subcarriers.

The frequency domain spreading method will now be described. FIG. 1 is aschematic diagram showing the state of digital symbols before OFDM-CDMAprocessing, and FIG. 2 is a schematic diagram showing the arrangement ofchips after OFDM-CDMA processing using frequency domain spreading. Withfrequency domain spreading, each of N digital symbols constituting aserial data sequence (FIG. 1) is multiplied by, for example, a spreadingcode with a spreading factor of M, the same value as the number ofsubcarriers M.

After spreading, the chips, arranged with M chips in parallel, undergoIFFT (inverse fast Fourier transform) processing sequentially, onesymbol at a time. As a result, N OFDM symbols for M subcarriers arecreated. That is to say, with frequency domain spreading, spread chipsare arranged on the frequency axis at their respective times (FIG. 2).In other words, spread chips are allocated to different subcarriers.

A sample configuration of a conventional OFDM-CDMA communicationapparatus that implements this frequency spreading method is shown inFIG. 3. First, transmitting system 2 of OFDM-CDMA communicationapparatus 1 will be described. In the OFDM-CDMA communication apparatus1, a plurality of transmit signals 1 through k, . . . , (4 k+1) through5 k are input to spreaders A1 through A(5 k) that perform spreadingprocessing using different spreading codes. The spread signals are addedby adders C1 through C5, as a result of which code division multiplexedsignals are obtained. In the case shown in FIG. 3, k transmit signalsare multicode-multiplexed by each of adders C1 through C5.

The code division multiplexed signals output from adders C1 through C5undergo parallel/serial conversion by a parallel/serial converter (P/S)4, and then undergo orthogonal frequency division multiplexing by meansof inverse fast Fourier transform processing by an inverse fast Fouriertransform circuit (IFFT) 5. By this means, an OFDM-CDMA signal is formedin which spread chips are distributed among a plurality of subcarriersthat have a mutually orthogonal relationship, and this OFDM-CDMA signalis transmitted via a radio transmitting section (RF) 10 that performsradio transmission processing such as digital/analog conversion andsignal amplification, and an antenna AN.

Next, receiving system 3 of OFDM-CDMA communication apparatus 1 will bedescribed. In OFDM-CDMA communication apparatus 1, an OFDM-CDMA signaltransmitted from an OFDM-CDMA communication apparatus with a similarconfiguration is input to a fast Fourier transform circuit (FFT) 6 viaan antenna AN and a radio receiving section (RF) 11 that performs radioreception processing such as analog/digital conversion. FFT 6 executesfast Fourier transform processing on the input signal, and therebyextracts a code division multiplexed signal distributed among aplurality of subcarriers.

A propagation path compensation circuit 7 compensates for phasefluctuations, etc., occurring in the propagation path, based on a knownsignal such as a propagation path estimation preamble included in thesignal. After propagation path compensation, the signal is despread by adespreader 8, and the received signal for that station is extracted fromthe spread plurality of transmit signals.

FIG. 4 shows the arrangement of OFDM-CDMA signals formed by OFDM-CDMAcommunication apparatus 1. As can be seen from FIG. 4, radiotransmitting apparatus divides 5 k transmit signals 1 through 5 k into 5groups, forms code division multiplexed signals on a group-by-groupbasis, and performs frequency domain spreading of the code divisionmultiplexed signals in subcarriers of different groups.

Specifically, code division multiplexed transmit signals 1 through k areallocated by frequency domain spreading to subcarriers #1 through #m,the same number as the spreading ratio m, code division multiplexedtransmit signals k+1 through 2 k are allocated by frequency domainspreading to subcarriers #4 m+1 through #5 m, and so on through to codedivision multiplexed transmit signals 4 k+1 through 5 k, which areallocated by frequency domain spreading to subcarriers #m+1 through #2m.

The number of subcarriers need not coincide with the spreading ratio.Here, a case has been shown in which subcarriers are divided into 5subcarrier groups, and the spreading ratio is made 1/5 the number ofsubcarriers in order for code division multiplexed signals to beallocated within each subcarrier group. However, the spreading ratio isnot limited to this case, and may be set arbitrarily.

In an OFDM-CDMA communication apparatus, it is necessary to increase thedegree of signal multiplexing in order to improve spectral efficiency.However, in a multipath environment, for instance, orthogonality betweenspreading codes is lost and error rate characteristics degrade. This isbecause multipathing occurs independently in each subcarrier, andtherefore inter-chip orthogonality is lost when each spread chip isspread along the frequency axis.

As the degree of signal multiplexing is increased, in particular,interference between spreading codes also increases, resulting ingreater degradation of error rate characteristics. Thus, a problem withconventional OFDM-CDMA communication apparatuses is the difficulty ofmaking spectral efficiency compatible with error rate characteristics.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an OFDM-CDMA radiotransmitting apparatus and radio transmission method that make itpossible to make spectral efficiency compatible with error ratecharacteristics.

This object is achieved by forming subcarriers to which spread signalsare allocated only in the frequency axis direction, and subcarriers towhich spread signals are allocated in both the frequency axis directionand the time axis direction, when performing radio transmission usingthe OFDM-CDMA method.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing showing the state of digital symbols beforeOFDM-CDMA processing;

FIG. 2 is a drawing showing the arrangement of chips after OFDM-CDMAprocessing using frequency domain spreading;

FIG. 3 is a block diagram showing a sample configuration of aconventional OFDM-CDMA communication apparatus;

FIG. 4 is a drawing showing the arrangement of OFDM-CDMA signals formedby a conventional OFDM-CDMA communication apparatus;

FIG. 5 is a schematic diagram showing a sample arrangement of subcarriergroups for which the degree of signal multiplexing has been reduced inan OFDM-CDMA signal of the present invention;

FIG. 6 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 1 of the presentinvention;

FIG. 7 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment2;

FIG. 8 is a drawing provided in order to explain an OFDM-CDMA signal forwhich the degree of signal multiplexing of subcarriers that include theDC point has been reduced in Embodiment 3;

FIG. 9 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment3;

FIG. 10 is a drawing provided in order to explain an OFDM-CDMA signalfor which the degree of signal multiplexing of subcarriers at a distancefrom the center frequency has been reduced in Embodiment 4;

FIG. 11 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment4;

FIG. 12 is a drawing of an OFDM-CDMA signal provided in order to explainthe side-lobe reduction effect according to Embodiment 4;

FIG. 13 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment5;

FIG. 14 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 6;

FIG. 15 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment7;

FIG. 16 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment8;

FIG. 17 is a drawing provided in order to explain cells;

FIG. 18 is a drawing showing a sample arrangement of an OFDM-CDMA signalwhen the cell to which the communicating-party communication terminalapparatus belongs is reported based on a subcarrier group for which thedegree of signal multiplexing has been reduced;

FIG. 19 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 9;

FIG. 20 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 10;

FIG. 21 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 11;

FIG. 22 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 12;

FIG. 23 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment13;

FIG. 24 is a drawing provided in order to explain an OFDM-CDMA signalwhen a subcarrier for which the degree of multiplexing has been reducedis located at intervals of a plurality of subcarriers in Embodiment 14;

FIG. 25 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment14;

FIG. 26 is a drawing provided in order to explain an OFDM-CDMA signalwhen a multiplex signal for which the degree of multiplexing has beenreduced is allocated to a plurality of subcarriers of a plurality ofgroups in Embodiment 14;

FIG. 27 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 15;

FIG. 28 is a block diagram showing the configuration of the receivingsystem of an OFDM-CDMA communication apparatus according to Embodiment16;

FIG. 29 is a drawing provided in order to explain an OFDM-CDMA signalaccording to Embodiment 17;

FIG. 30 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 17;

FIG. 31 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment18;

FIG. 32 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment19;

FIG. 33 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment20;

FIG. 34 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment21;

FIG. 35 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment22;

FIG. 36 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment23;

FIG. 37 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment24;

FIG. 38 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment25;

FIG. 39 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment26;

FIG. 40 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment27;

FIG. 41 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment28;

FIG. 42 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment29;

FIG. 43 is a drawing provided in order to explain an OFDM-CDMA signalaccording to Embodiment 30;

FIG. 44 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment30;

FIG. 45 is a drawing provided in order to explain an OFDM-CDMA signalaccording to Embodiment 31;

FIG. 46 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment31;

FIG. 47 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment32;

FIG. 48 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment33;

FIG. 49 is a drawing provided in order to explain an OFDM-CDMA signalaccording to Embodiment 33;

FIG. 50 is a drawing provided in order to explain an OFDM-CDMA signalaccording to Embodiment 34;

FIG. 51 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 34;

FIG. 52 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 35;

FIG. 53 is a block diagram showing the configuration of the receivingsystem of an OFDM-CDMA communication apparatus according to Embodiment36;

FIG. 54 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment37;

FIG. 55 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment38;

FIG. 56 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment39;

FIG. 57 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment40;

FIG. 58 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment41;

FIG. 59 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment42;

FIG. 60 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment43;

FIG. 61 is a drawing showing sample propagation path estimation preamblearrangements according to Embodiment 44;

FIG. 62 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment44;

FIG. 63 is a block diagram showing the configuration of an OFDM-CDMAcommunication apparatus according to Embodiment 45;

FIG. 64 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment46;

FIG. 65 is a drawing provided in order to explain an OFDM-CDMA signalaccording to Embodiment 46;

FIG. 66 is a drawing provided in order to explain an OFDM-CDMA signalaccording to Embodiment 47; and

FIG. 67 is a block diagram showing the configuration of the transmittingsystem of an OFDM-CDMA communication apparatus according to Embodiment47.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the accompanying drawings, embodiments of thepresent invention will be explained in detail below.

Embodiment 1

In an OFDM-CDMA radio transmitting apparatus according to Embodiment 1of the present invention, the degree of signal multiplexing is setindividually for each subcarrier. For example, the spreading ratio maybe set as 1/5 the number of subcarriers, with all subcarriers beingdivided into 5 subcarrier groups. Here, as shown in FIG. 5, the degreeof signal multiplexing is set low for one subcarrier group G1 (indicatedby shading in FIG. 5) of the 5 subgroups.

As a result, within the 5 subcarrier groups, error rate characteristicscan be improved for subcarrier group G1 without lowering spectralefficiency at all for the subcarrier groups other than subcarrier groupG1. By this means, it is possible to make spectral efficiency compatiblewith error rate characteristics.

In this embodiment, a signal for a communicating party whose channelquality is poor, such as a distant radio station or a radio station witha poor SIR (Signal to Interference Ratio), for example, is allocated tosubcarrier group G1 for which the degree of signal multiplexing is setlow. By this means, it is possible to improve the error ratecharacteristics of a communicating party with poor channel quality, withalmost no lowering of spectral efficiency.

Here, a case has been described in which the spreading ratio is made 1/5the number of subcarriers, but the spreading ratio is not restricted tothis case, and can be set arbitrarily. For example, it is possible todivide all the subcarriers into 4 subcarrier groups, and to set thespreading ratio of one of those subcarrier groups at twice that of theother groups.

Thus, according to this embodiment, by setting the degree of signalmultiplexing individually for each subcarrier and reducing the degree ofsignal multiplexing for a user with poor channel quality, it is possibleto improve error rate characteristics with almost no lowering ofspectral efficiency.

The actual configuration of an OFDM-CDMA communication apparatusaccording to this embodiment will now be described, using FIG. 6.Transmitting system 101 of OFDM-CDMA communication apparatus 100 dividesn transmit signals 1 through n into 5 groups, and performs frequencydomain spreading of transmit signals within each group onto the sameplurality of subcarriers.

In actuality, in transmitting system 101, transmit signals 1 through nare input to spreaders A1 through An, which perform spreading processingusing different spreading codes. The spread signals are added by addersB1 through B5 as signal multiplexing degree selecting sections, one ofwhich is provided for each group (in this embodiment, each of 5 groups),and by this means, code division multiplexed signals S1 through S5corresponding to the predetermined number of groups are obtained.

Here, in transmitting system 101, adders B1 through B4 form codedivision multiplexed signals S1 through S4 in each of which k transmitsignals are multiplexed, whereas adder B5 forms code divisionmultiplexed signal S5 in which fewer than k transmit signals aremultiplexed. That is to say, the number of signals (n−4 k) of transmitsignals (4 k+1) through n that are code division multiplexed by adder B5is selected so that 1<(n−4 k)<k. By this means, code divisionmultiplexed signals S1 through S4, and S5, with different chip rates,are output by adders B1 through B4 and adder B5.

Code division multiplexed signals S1 through S5 obtained by adders B1through B5, respectively, are input to a parallel/serial converter (P/S)102 as a multiplex signal allocation selecting section. Parallel/serialconverter 102 rearranges code division multiplexed signals S1 through S5in a predetermined order, and outputs them as a serial signal S6. Inthis embodiment, this arrangement order determines which subcarriergroups in FIG. 5 code division multiplexed signals S1 through S5 areallocated to by frequency domain spreading.

Serial signal S6 output from parallel/serial converter 102 is input toan inverse fast Fourier transform circuit (IFFT) 103 as an orthogonalfrequency division multiplexing section. Inverse fast Fourier transformcircuit 103 executes inverse fast Fourier transform processing on serialsignal S6 for each of code division multiplexed signals S1 through S5,and thereby allocates spread chips by distributing them among aplurality of subcarriers that are in a mutually orthogonal relationship.

At this time, code division multiplexed signal S1 that was code divisionmultiplexed by adder B1, for example, is allocated by frequency domainspreading to a certain subcarrier group, and code division multiplexedsignal S5 that was code division multiplexed by adder B5 is allocated byfrequency domain spreading to subcarrier group G1 in FIG. 5.

In this way, it is possible to form an OFDM-CDMA signal S7 in which atransmit signal with a smaller degree of multiplexing than othersubcarrier groups is allocated to subcarrier group G1. ObtainedOFDM-CDMA signal S7 is then transmitted via a radio transmitting section(RF) 104 that performs radio transmission processing such asdigital/analog conversion and signal amplification, and an antenna AN.

Next, receiving system 110 of OFDM-CDMA communication apparatus 100 willbe described. In OFDM-CDMA communication apparatus 100, an OFDM-CDMAsignal transmitted from an OFDM-CDMA communication apparatus with asimilar configuration is input to a fast Fourier transform circuit (FFT)111 via antenna AN and a radio receiving section (RF) 114 that performsradio reception processing such as analog/digital conversion. FFT 111executes fast Fourier transform processing on the input signal, andthereby extracts a code division multiplexed signal distributed among aplurality of subcarriers.

A propagation path compensation circuit 112 compensates for phasefluctuations, etc., occurring in the propagation path, based on a knownsignal such as a propagation path estimation preamble included in thesignal. After propagation path compensation, the signal is despread by adespreader 113, and the received signal for that station is extractedfrom the spread plurality of transmit signals.

With the above configuration, transmit signals (4 k+1) through nallocated to subcarriers with a low degree of signal multiplexing aresubject to less inter-code interference on the propagation path thantransmit signals 1 through k, . . . , (3 k+1) through 4 k allocated tosubcarriers with a high degree of multiplexing.

As a result, compared with the case in which the degree of signalmultiplexing is decided uniformly for all subcarriers, if transmitsignals (4 k+1) through n carrying important information or for which itis desired to improve the error rate are allocated to subcarriers with alow degree of signal multiplexing, and transmit signals 1 through k, . .. , (3 k+1) through 4 k for which the error rate need not be improved somuch are allocated to subcarriers with a high degree of signalmultiplexing, degradation of error rate characteristics can be preventedwithout lowering spectral efficiency significantly.

Thus, an OFDM-CDMA communication apparatus 100 can be implemented thatenables spectral efficiency and error rate characteristics to be madecompatible.

Embodiment 2

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 2 of the present invention is that, by selecting as atransmit signal for which the degree of signal multiplexing is set low,described in Embodiment 1, a transmit signal such as retransmissioninformation or control information for which better channel quality isrequired than for other data, the quality of data for which betterchannel quality is required than for other data is improved.

FIG. 7 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In OFDM-CDMA communication apparatus 200according to this embodiment, the special feature lies particularly inthe transmitting system 201, and therefore a description of thereceiving system will be omitted. In FIG. 7, in which partscorresponding to those in FIG. 6 are assigned the same codes as in FIG.6, OFDM-CDMA communication apparatus 200 transmits retransmissionsignals 1 through m as transmit signals for which the degree of signalmultiplexing is set low.

That is to say, number of signals m of retransmission signals 1 throughm is selected so that 1<m<k. As a result, code division multiplexedretransmission signals 1 through m are allocated by frequency domainspreading to subcarrier group G1 with a low degree of signalmultiplexing shown in FIG. 5, thereby enabling the error ratecharacteristics of those retransmission signals 1 through m to beimproved.

In performing retransmission as referred to here, the propagation pathenvironment between the local station and the far-end station are oftenpoor, and as the number of retransmissions increases the same signalmust be transmitted numerous times, resulting in lower transmissionefficiency. In this embodiment, the error rate characteristics ofretransmission signals 1 through m can be improved, making it possibleto reduce the number of retransmissions.

Signals allocated to subcarriers for which the degree of signalmultiplexing is set low are not limited to retransmission signals, andcontrol signals may also be effectively allocated to such subcarriers.For example, if the quality of a control signal degrades, it may nolonger be possible to establish communication. According to thisembodiment, this can be prevented effectively.

According to the above configuration, by allocating data such asretransmission information or control information, for which betterchannel quality is required than for other data, to subcarriers forwhich the degree of signal multiplexing is set low, the quality of datafor which better channel quality is required than for other data can beimproved with almost no lowering of spectral efficiency.

Embodiment 3

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 3 of the present invention is that, in addition to providingthe configuration of Embodiment 1, by reducing the degree of signalmultiplexing of subcarriers that include the DC (Direct Current) point,as shown in FIG. 8, the error rate characteristics of the subcarrierlocated at the DC point are improved.

Generally, in an OFDM-CDMA radio transmitting apparatus, DC offset isgenerated by analog circuitry provided in the amplifier of the radiotransmitting section (RF) 104, and therefore the error ratecharacteristics of a signal transmitted by subcarriers near the DC pointare poorer than those of signals transmitted by other subcarriers.

Focusing on this point, this embodiment improves the error ratecharacteristics of a subcarrier located at the DC point by reducing thedegree of signal multiplexing of subcarriers that include the DC point.

In FIG. 9, in which parts corresponding to those in FIG. 6 are assignedthe same codes as in FIG. 6, transmitting system 301 of OFDM-CDMAcommunication apparatus 300 according to this embodiment has a similarconfiguration to that of transmitting system 101 in FIG. 6 except forthe configuration of parallel/serial converter (P/S) 302 as a multiplexsignal allocation selecting section.

That is to say, parallel/serial converter 302 forms a serial signal S10in which code division multiplexed signals S1 through S5 are rearrangedin an order whereby code division multiplexed signal S5 with a lowdegree of signal multiplexing, output from adder B5, is allocated tosubcarrier group G2 that includes the DC point, as shown in FIG. 8, andsends this to next-stage inverse fast Fourier transform circuit 103. Bythis means, an OFDM-CDMA signal S11 is obtained from inverse fastFourier transform circuit 103 such that code division multiplexed signalS5 with a low degree of signal multiplexing is allocated to subcarriergroup G2 that includes the DC point, as shown in FIG. 8.

According to the above configuration, the error rate characteristics ofsubcarriers including the DC point can be improved by reducing thedegree of signal multiplexing of the subcarrier located at the DC point.

Embodiment 4

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 4 of the present invention is that, in addition to providingthe configuration of Embodiment 1, by reducing the degree of signalmultiplexing of subcarriers at a distance from the center frequency, asshown in FIG. 10, error rate characteristics in the event of adjacentchannel interference signals or analog filter degradation are improved.

When there are adjacent channel interference signals, the further asubcarrier is from the center frequency, the greater is theinterference, and therefore the further a subcarrier is from the centerfrequency, the greater is the degradation of error rate characteristics.In addition, degradation (amplitude deviation or phase deviation) of ananalog filter provided in the latter-stage radio transmitting section(RF) 104 is also greater the further a subcarrier is from the centerfrequency.

Focusing on this point, this embodiment improves error ratecharacteristics in the event of adjacent channel interference signals oranalog filter degradation by reducing the degree of signal multiplexingof subcarriers at a distance from the center frequency.

In FIG. 11, in which parts corresponding to those in FIG. 6 are assignedthe same codes as in FIG. 6, transmitting system 401 of OFDM-CDMAcommunication apparatus 400 according to this embodiment has a similarconfiguration to that of transmitting system 101 in FIG. 6 except forthe configuration of parallel/serial converter (P/S) 402 as a multiplexsignal allocation selecting section.

That is to say, parallel/serial converter 402 forms a serial signal S6in which code division multiplexed signals S1 through S5 are rearrangedin an order whereby code division multiplexed signal S5 with a lowdegree of signal multiplexing, output from adder B5, is allocated tosubcarrier groups G3 and G4 that are at a distance from the centerfrequency, as shown in FIG. 10, and sends this to next-stage inversefast Fourier transform circuit 103. By this means, an OFDM-CDMA signalS7 is obtained from inverse fast Fourier transform circuit 103 such thatcode division multiplexed signal S5 with a low degree of signalmultiplexing is allocated to subcarrier groups G3 and G4 that are at adistance from the center frequency, as shown in FIG. 10.

According to the above configuration, error rate characteristics in theevent of adjacent channel interference signals or analog filterdegradation can be improved by reducing the degree of signalmultiplexing of subcarriers at a distance from the center frequency.

Also, out-of-band side lobes occur in OFDM-CDMA, and as shown in FIG.12, the side lobe component is large for subcarriers at a distance fromthe center frequency. By reducing the degree of signal multiplexing ofsubcarriers at a distance from the center frequency in this case, as inthis embodiment, the transmission power of subcarriers at a distancefrom the center frequency can be decreased, and thus side lobes can alsobe decreased. As a result, according to the configuration of thisembodiment, out-of-band leakage power can also be decreased.

Embodiment 5

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 5 of the present invention is that error rate characteristicsin the event of adjacent channel interference signals or analog filterdegradation are further improved by raising the transmission power ofsubcarriers for which the degree of signal multiplexing is set low abovethat of other subcarriers, in addition to reducing the degree of signalmultiplexing of subcarriers at a distance from the center frequency inthe same way as in Embodiment 4.

Subcarriers for which the degree of signal multiplexing is set low havelower transmission power than other subcarriers. Therefore, transmissionpower is further raised, enabling error rate characteristics in theevent of adjacent channel interference signals or analog filterdegradation to be further improved.

Also, even if transmit signal power is increased to a certain extent forsubcarriers for which the degree of signal multiplexing is set low,compared with other subcarriers, there is little likelihood of theoccurrence of a peak voltage greater than or equal to a predeterminedvalue—which is a problem with the OFDM-CDMA method-and therefore errorrate characteristics in the event of adjacent channel interferencesignals or analog filter degradation can be effectively improved.

In FIG. 13, in which parts corresponding to those in FIG. 11 areassigned the same codes as in FIG. 11, transmitting system 501 ofOFDM-CDMA communication apparatus 500 according to this embodiment has asimilar configuration to that of transmitting system 401 in FIG. 11, butdiffers in having a multiplier 502 that increases the signal value of acode division multiplexed signal with a low degree of signalmultiplexing. If a value greater than 1 is selected as themultiplication coefficient of multiplier 502, the signal power of atransmit signal allocated to subcarriers with a low degree of signalmultiplexing can be increased.

According to the above configuration, by reducing the degree of signalmultiplexing of subcarriers at a distance from the center frequency, andalso raising the signal power of a signal for which the degree of signalmultiplexing is set low above the signal power of transmit signalsallocated to other subcarriers, in addition to achieving the effect ofEmbodiment 4 it is possible to significantly improve error ratecharacteristics in the event of adjacent channel interference signals oranalog filter degradation.

Embodiment 6

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 6 of the present invention is that error rate characteristicsof a signal for which the degree of signal multiplexing is set low arefurther improved by making the spreading ratio of subcarriers for whichthe degree of signal multiplexing is set low larger than the spreadingratio of other subcarriers.

That is to say, the larger the spreading ratio, the longer is the taplength of spread chips for one symbol, enabling despreading accuracy tobe increased accordingly, and transmit symbols to be accuratelyrecovered on the receiving side.

In FIG. 14, in which parts corresponding to those in FIG. 6 are assignedthe same codes as in FIG. 6, transmitting system 601 of OFDM-CDMAcommunication apparatus 600 has a similar configuration to that oftransmitting system 101 described in Embodiment 1, but differs in thatthe spreading ratio of spreaders E(4 k+1) through En, which despreadtransmit signals (4 k+1) through n for which the degree of signalmultiplexing is set low, is made larger than the spreading ratio ofspreaders A1 through A (4 k), which despread the other transmit signals1 through 4 k.

In receiving system 610 of OFDM-CDMA communication apparatus 600, asignal that has undergone propagation path compensation is input to aselecting section 611 as a multiplex signal discrimination section.Selecting section 611 divides the sequentially input signal into a codedivision multiplexed signal with a high degree of signal multiplexingand a code division multiplexed signal with a low degree of signalmultiplexing, and outputs these signals. This discrimination processingcan be performed easily if, for example, the input signal is split inthe same order as in the code division multiplexed signal rearrangementprocessing by parallel/serial converter 102 of transmitting system 601.This rearrangement order is assumed to have been determined beforehandbetween the respective radio stations. Discrimination can also becarried out based on the chip rate, etc., of the input code divisionmultiplexed signal.

By this means, a code division multiplexed signal with a low degree ofsignal multiplexing is input to a despreader 613 for which the spreadingratio is set to a large value in the same way as in transmitting system601, and the received signal for this station is extracted from the codedivision multiplexed signal by despreading processing. Also, a codedivision multiplexed signal with a high degree of signal multiplexing isinput to a despreader 612, and the received signal for this station isextracted from the code division multiplexed signal by despreadingprocessing.

In the above configuration, the spreading ratio for a signal allocatedto subcarriers for which the degree of signal multiplexing is set lowmay, for example, be made twice the spreading ratio of signals allocatedto other subcarriers. By doubling the spreading ratio, thesignal-to-noise ratio can also be doubled, and it is therefore possibleto significantly improve the quality of data such as control informationor retransmission information for which better quality is required thanfor other data, or the quality of a user with poor quality. Thespreading ratio here is not limited to twice the spreading ratio ofother subcarriers, but can be set arbitrarily.

According to the above configuration, by making the spreading ratio of asignal allocated to subcarriers for which the degree of signalmultiplexing is set low larger than the spreading ratio of signalsallocated to other subcarriers, in addition to achieving the effect ofEmbodiment 1 it is possible to significantly improve the quality of datasuch as control information or retransmission information for whichbetter quality is required than for other data, or the quality of datafor a user with poor quality.

Embodiment 7

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 7 of the present invention is that signals in which a knownsignal is spread are multiplexed in subcarriers for which the degree ofsignal multiplexing is set low.

By multiplexing signals in which a known signal (generally called apilot signal) is spread, it is possible for the receiving side toperform residual phase error detection, cell identification in amulti-cell system, and so forth, based on this known signal. Accordingto this embodiment, the accuracy of such processing can be improved.

While a pilot signal is also used in DS-CDMA, when used in OFDM-CDMA afrequency diversity effect is obtained, enabling the accuracy of theabove-mentioned processing to be significantly improved compared withDS-CDMA.

FIG. 15 shows the actual configuration of transmitting system 701 of anOFDM-CDMA communication apparatus 700 according to this embodiment. InFIG. 15, in which parts corresponding to those in FIG. 6 are assignedthe same codes as in FIG. 6, transmitting system 701 of OFDM-CDMAcommunication apparatus 700 has a similar configuration to that oftransmitting system 101 described in Embodiment 1, but differs in that aknown signal is included in transmit signals (4 k+1) through n with alow degree of multiplexing.

According to the above configuration, a known signal used forpropagation path compensation, synchronization processing, or the like,on the receiving side is allocated to subcarriers for which the degreeof signal multiplexing is set low, enabling known signal error ratecharacteristics to be improved, and making it possible to performreceived signal propagation path compensation and synchronizationprocessing with a high degree of accuracy, and to obtain a high-qualityreceived signal.

Embodiment 8

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 8 of the present invention is that, by steering subcarriersfor which the degree of signal multiplexing is set low, the occurrenceof burst errors is prevented in a signal with a low degree of signalmultiplexing when channel fluctuations are slow.

Even when the degree of signal multiplexing of particular subcarriershas been reduced in order to improve the error rate characteristics ofthose signals, if channel fluctuations are slow, there is a risk ofburst errors occurring in signals allocated to those subcarriers due tofrequency selective fading.

Focusing on this point, this embodiment steers subcarriers for which thedegree of signal multiplexing is set low. By this means, it is possibleto prevent a poor state from continuing for a long period forsubcarriers with a low degree of signal multiplexing, and to prevent theoccurrence of burst errors in data for which good quality is required.To explain this using FIG. 5, the subcarriers with a low degree ofsignal multiplexing indicated by shading in FIG. 5 are taken to besubcarriers of a frequency lower than the center frequency, but it isalso possible for subcarriers with a low degree of signal multiplexingto be, for example, sequentially close to the center frequency and tohave a frequency higher than the center frequency.

In FIG. 16, in which parts corresponding to those in FIG. 15 areassigned the same codes as in FIG. 15, transmitting system 801 ofOFDM-CDMA communication apparatus 800 according to this embodiment has asimilar configuration to that of transmitting system 701 in FIG. 15except for the configuration of parallel/serial converter (P/S) 802 as amultiplex signal allocation selecting section.

That is to say, parallel/serial converter 802 steers subcarriers with alow degree of signal multiplexing in OFDM-CDMA signal S7 formed bynext-stage inverse fast Fourier transform circuit 103, by switching theoutput order of code division multiplexed signal S5 with a low degree ofsignal multiplexing, input from adder B5, with the order of the othercode division multiplexed signals S1 through S4.

At this time, each time a signal indicating a transmit symbol number isinput, parallel/serial converter 802 switches the output order of codedivision multiplexed signals S1 through S5, and by this means thereceiving side can recover received symbols satisfactorily.

According to the above configuration, by steering subcarriers with a lowdegree of signal multiplexing, the occurrence of burst errors whenchannel fluctuations are slow can be prevented.

Embodiment 9

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 9 of the present invention is that, by changing subcarrierswith a low degree of signal multiplexing according to the cell, it ispossible for cell identification to be performed by acommunicating-party communication terminal apparatus without increasingthe number of spreading codes assigned to a known signal.

A case will be considered in which, for example, the number of cells is7, as shown in FIG. 7. All subcarriers are divided into 7 groups, andsubcarriers with a low degree of signal multiplexing are assigned on acell-by-cell basis. For example, in order to notify a communicationterminal apparatus that communication terminal apparatus belongs to cell4, a signal with a low degree of signal multiplexing can be allocated tothe subcarrier group corresponding to cell 4, as shown in FIG. 18.

As a result, the receiving side can easily identify the cell to whichthe station itself belongs by detecting to which subcarrier group asignal with a low degree of signal multiplexing is allocated.

FIG. 19 shows the configuration of an OFDM-CDMA communication apparatus900 according to this embodiment. In FIG. 19, in which partscorresponding to those in FIG. 14 or FIG. 16 are assigned the same codesas in FIG. 14 or FIG. 16, transmitting system 901 of OFDM-CDMAcommunication apparatus 900 has a similar configuration to that oftransmitting system 801 in FIG. 16 except for the configuration ofparallel/serial converter 902 as a multiplex signal allocation selectingsection.

Parallel/serial converter 902 sends code division multiplexed signal S5with a low degree of signal multiplexing in an order such that codedivision multiplexed signal S5 with a low degree of signal multiplexingis allocated to the subcarrier group at the location corresponding tothe cell to which the communicating-party communication terminalapparatus belongs, based on a signal indicating the group in which aknown signal is inserted (that is, a signal indicating a group with alow degree of signal multiplexing).

In receiving system 910, a signal that has undergone propagation pathcompensation is divided into a code division multiplexed signal with alow degree of signal multiplexing and a code division multiplexed signalwith a high degree of signal multiplexing by a selecting section 611 asa multiplex signal selecting section. The code division multiplexedsignal with a low degree of signal multiplexing is then subjected todespreading processing by a despreader 613.

In addition, the code division multiplexed signal with a low degree ofsignal multiplexing is subjected to despreading processing by adespreader 911 that has the spreading code of the known signal as acoefficient. The timing at which the maximum value of the despreadsignal is obtained is detected by a maximum value detection circuit 912.Based on this maximum value detection timing, the subcarrier group towhich the known signal (that is, the signal with a low degree of signalmultiplexing) is allocated is ascertained, and therefore OFDM-CDMAcommunication apparatus 900 can identify the cell to which this stationbelongs.

According to the above configuration, by changing subcarriers with a lowdegree of signal multiplexing according to the cell, it is possible forthe receiving side to identify easily the cell to which it belongs, evenif a signal for cell identification is not transmitted. As a result, anOFDM-CDMA communication apparatus 900 can be implemented that, inaddition to achieving the effect obtained in above-described Embodiment1, improves transmission efficiency inasmuch as a signal for cellidentification need not be transmitted.

Embodiment 10

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 10 of the present invention is that the quality of importantinformation can be further improved by making the modulation M-ary valueof subcarriers with a low degree of signal multiplexing smaller than forother subcarriers. Incidentally the modulation method can be setarbitrarily.

In FIG. 20, in which parts corresponding to those in FIG. 6 are assignedthe same codes as in FIG. 6, in OFDM-CDMA communication apparatus 1000,of modulation circuits F1 through Fn provided in correspondence totransmit signals 1 through n in transmitting system 1001, modulationcircuits F(4 k+1) through Fn provided in correspondence to transmitsignals (4 k+1) through n, for which the degree of signal multiplexingis low, modulate transmit signals (4 k+1) through n by means of amodulation method with a smaller modulation M-ary value than that ofmodulation circuits F1 through F(4 k) provided in correspondence totransmit signals 1 through 4 k, for which the degree of signalmultiplexing is high. For example, modulation circuits F1 through F(4 k)may execute 16QAM (Quadrature Amplitude Modulation) modulationprocessing, while modulation circuits F(4 k+1) through Fn execute QPSK(Quadrature Phase Shift Keying) modulation processing.

In receiving system 1010, a signal that has undergone propagation pathcompensation is divided into a code division multiplexed signal with ahigh degree of signal multiplexing and a code division multiplexedsignal with a low degree of signal multiplexing by a selecting section611. The code division multiplexed signal with a low degree of signalmultiplexing is sent to despreader 613, by which the signal for thisstation is extracted.

Decoding circuit 1012 performs the reverse of the processing bymodulation circuits F(4 k+1) through Fn, thereby demodulating themodulated signal for which the modulation M-ary value is small, andobtaining a received signal. In the case of the code divisionmultiplexed signal with a high degree of signal multiplexing, on theother hand, the signal for this station is extracted by despreader 612,and decoding circuit 1011 performs the reverse of the processing bymodulation circuits F1 through F(4 k), thereby demodulating themodulated signal for which the modulation M-ary value is large, andobtaining a received signal.

According to the above configuration, in addition to provision of theconfiguration in Embodiment 1, by making the modulation M-ary value of atransmit signal correspondingly smaller for a multiplex signal with alow degree of multiplexing, it is possible, in addition to achieving theeffect obtained in Embodiment 1, to significantly improve the error ratecharacteristics of a multiplex signal with a low degree of multiplexing.

This principle is not limited to OFDM-CDMA, but is also effective whenapplied to OFDM. That is to say, in OFDM, the modulation M-ary value ofspecific subcarriers is made smaller than the modulation M-ary value ofthe other subcarriers. For example, by making the modulation M-ary valueof signals allocated to subcarriers for which degradation is greaterthan for other subcarriers, such as a subcarrier at the DC point andsubcarriers on either side thereof, it is possible to improve error ratecharacteristics without greatly lowering transmission efficiency.Specifically, this can be implemented with a configuration virtually thesame as the configuration of transmitting system 1001 in FIG. 20 withthe exception of the spreaders and adders. By this means, it is possibleto greatly reduce the circuit scale compared with an OFDM-CDMA system,and to improve the error rate characteristics of specific subcarriers.

Also, with OFDM, a one-times spreading, one-code multiplexing CDMA-OFDMmethod, as it is called, is possible. That is to say, OFDM is aCDMA-OFDM method whereby one-times spreading is performed by a spreadingsection, and the degree of signal multiplexing for transmission by eachsubcarrier is made 1 by a signal multiplexing degree selecting section.

Embodiment 11

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 11 of the present invention is that, by changing thespreading code of a known signal only at the start of a frame, framesynchronization can easily be achieved using the result of despreadingthe known signal.

Generally, achieving frame synchronization requires processing such asthe insertion of a special preamble at the start of a frame, but in thisembodiment, frame synchronization can easily be achieved without theneed for such processing.

FIG. 21 shows the configuration of an OFDM-CDMA communication apparatus1100 according to this embodiment. In FIG. 21, in which partscorresponding to those in FIG. 19 are assigned the same codes as in FIG.19, transmitting system 1101 of OFDM-CDMA communication apparatus 1100has a similar configuration to that of transmitting system 901 in FIG.19, but differs in that known signal 1 is spread using a differentspreading code only when a signal indicating the start of a frame isinput to spreader G (n−1) that spreads known signal 1.

In receiving system 1110, a code division multiplexed signal with a lowdegree of signal multiplexing, separated by selecting section 611, isinput to despreaders 911 and 1111. In the same way as described inEmbodiment 9, despreader 911 executes despreading processing on an inputsignal with a spreading code other than that for the start of a frameused by spreader G(n−1) in despreader 911 as a coefficient, and the cellto which this station belongs is identified by having maximum valuedetection circuit 912 detect the maximum value timing of the signal thathas undergone despreading.

Despreader 1111 performs despreading processing on an input signal,using the spreading code used by spreader G(n−1) at the start of aframe. Maximum value detection circuit 1112 obtains a framesynchronization signal by detecting the maximum value of the outputresult of despreader 1111.

According to the above configuration, by changing the spreading code ofa known signal only at the timing of the start of a transmit frame,frame synchronization can be performed easily without inserting a signalfor frame synchronization such as a preamble. As a result, it is nolonger necessary to insert a signal for frame synchronization such as apreamble. Therefore, the configuration can be simplified accordingly,and transmission efficiency can be improved in as much as a signal forframe synchronization such as a preamble need not be transmitted.

In the above-described embodiment, a case has been described in whichthe spreading code of a known signal is changed at the timing of thestart of a transmit frame, but it is also possible to change the type ofsignal transmitted from known signal 1 to known signal 2 at the timingof the start of a transmit frame, as shown in FIG. 21. If this is done,the same effect as in the above-described embodiment can be obtained byexecuting despreading processing using a coefficient corresponding toknown signal 2 in despreader 1111.

Embodiment 12

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 12 of the present invention is that, by multiplexing a signalin which a known signal is spread only at the start of a frame, framesynchronization can easily be achieved without increasing the number ofspreading codes assigned to a known signal.

When the spreading code for the start of a frame is changed, as inEmbodiment 11, it is necessary to assign a plurality of spreading codesto a known signal, and therefore the number of spreading codes neededincreases accordingly. However, in this embodiment, framesynchronization can easily be achieved without increasing the necessarynumber of spreading codes.

FIG. 22 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 22, in which parts correspondingto those in FIG. 19 are assigned the same codes as in FIG. 19,transmitting system 1201 of OFDM-CDMA communication apparatus 1200 has asimilar configuration to that of transmitting system 901 in FIG. 19, butdiffers in that a known signal is input to spreader A(n−1) via an on/offswitch (ON/OFF) 1202. On/off switch 1202 inputs the known signal tospreader A(n−1) only when a signal indicating the start of a frame isinput. Thus, transmitting system 1201 multiplexes and transmits a signalin which a known signal is spread only at the start of a frame.

In receiving system 1210, a code division multiplexed signal with a lowdegree of signal multiplexing, separated by selecting section 611, isinput to despreader 911. The signal despread by despreader 911 using thesame spreading code as spreader A(n−1) is sent to maximum valuedetection circuits 912 and 1211.

Maximum value detection circuit 912 identifies the cell to which thisstation belongs by detecting the maximum value timing of the signal thathas undergone despreading. Maximum value detection circuit 1211 obtainsa frame synchronization signal by detecting the maximum value timing ofthe signal that has undergone despreading (the detection timing of themaximum value of the correlation result for one frame).

According to the above configuration, by multiplexing a signal in whicha known signal is spread only at the start of a frame, framesynchronization can easily be achieved without increasing the number ofspreading codes assigned to a known signal, in contrast to Embodiment11.

Embodiment 13

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 13 of the present invention is that the quality of specificdata is improved by assigning a plurality of spreading codes to aspecific signal, and transmitting the signal after spreading with aplurality of spreading codes.

FIG. 23 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 23, in which parts correspondingto those in FIG. 15 are assigned the same codes as in FIG. 15,transmitting system 1301 of OFDM-CDMA communication apparatus 1300 has asimilar configuration to that of transmitting system 701 in FIG. 15, butdiffers in that transmit signal n is spread by spreaders A(n−1) and Anusing different spreading codes.

According to the above configuration, by assigning a plurality ofspreading codes to a specific signal, and transmitting the signal afterspreading with a plurality of spreading codes, the receiving side canrecover the specific signal with a high signal level by despreading thespecific signal using a plurality of spreading codes, and combining thesignals after despreading processing. As a result, error ratecharacteristics can be significantly improved for that specific signal.

Embodiment 14

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 14 of the present invention is that quality in a propagationenvironment in which multipath delay time is short is further improvedby allocating a code division multiplexed signal for which the degree ofsignal multiplexing is set low at intervals of a plurality ofsubcarriers, as indicated by shading in FIG. 24.

In a propagation environment in which multipath delay time is short, itmay happen that the reception levels of a plurality of subcarriersdecline together. In this case, the effectiveness of error correctiondecreases, and channel quality greatly degrades.

Focusing on this point, this embodiment prevents the reception levels ofcode division multiplexed signals for which the degree of signalmultiplexing is set low from declining together by allocating codedivision multiplexed signals for which the degree of signal multiplexingis set low to, for example, every second subcarrier. As a result,quality can be further improved in a propagation environment in whichmultipath delay time is short.

FIG. 25 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 25, in which parts correspondingto those in FIG. 16 are assigned the same codes as in FIG. 16,transmitting system 1401 of OFDM-CDMA communication apparatus 1400 has asimilar configuration to that of transmitting system 801 in FIG. 16except for the configuration of parallel/serial converter 1402 as amultiplex signal allocation selecting section.

Parallel/serial converter 1402 rearranges as appropriate code divisionmultiplexed signals S1 through S5 input from adders B1 through B5, basedon a signal indicating the group to which a known signal belongs (thatis, the group on which addition is performed by adder B5), so that codedivision multiplexed signals for which the degree of signal multiplexingis set low are allocated at intervals of a plurality of subcarriers, andsends the resulting signal to inverse fast Fourier transform circuit103.

According to the above configuration, the error rate characteristics ofa code division multiplexed signal with a low degree of signalmultiplexing can be significantly improved by allocating a code divisionmultiplexed signal for which the degree of signal multiplexing is setlow at intervals of a plurality of subcarriers.

Embodiment 15

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 15 of the present invention is that a code divisionmultiplexed signal for which the degree of signal multiplexing is setlow is allocated to a plurality of subcarrier groups G5 and G6, asindicated by shading in FIG. 26.

By this means, by having the receiving side select from subcarriergroups G5 and G6, after propagation path compensation, the one with thehigher reception level, it is possible to prevent a fall in thereception level of code division multiplexed signals for which thedegree of signal multiplexing is set low, and to significantly improvereception quality.

FIG. 27 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 27, in which parts correspondingto those in FIG. 19 are assigned the same codes as in FIG. 19,transmitting system 1501 of OFDM-CDMA communication apparatus 1500 has asimilar configuration to that of transmitting system 901 in FIG. 19except for the configuration of parallel/serial converter 1502 as amultiplex signal allocation selecting section.

Parallel/serial converter 1502 rearranges as appropriate code divisionmultiplexed signals S1 through S5 into an order whereby code divisionmultiplexed signal S5 is allocated to subcarrier groups G5 and G6, asshown in FIG. 26, and sends the resulting signal to inverse fast Fouriertransform circuit 103.

In receiving system 1510, the reception levels of subcarrier groups G5and G6 are detected by propagation path compensation circuit 112 basedon the reception level of a known signal, and the results of thisdetection are sent to a selecting section 1511. Selecting section 1511selects, from the two subcarrier groups G5 and G6 for code divisionmultiplexed signals with a low degree of signal multiplexing input fromselecting section 611, only the subcarrier group with the higherreception level, and sends this to next-stage despreaders 613 and 911.The subsequent processing is the same as in Embodiment 9.

According to the above configuration, reception quality can besignificantly improved by transmitting a multiplex signal with a lowdegree of signal multiplexing allocated to a plurality of subcarriergroups, and demodulating only the multiplex signal of a subcarrier groupwith a small decrease in reception level on the receiving side.

Embodiment 16

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 16 of the present invention is that, whereas in Embodiment 15a multiplex signal with a low degree of signal multiplexing istransmitted allocated to a plurality of subcarrier groups, and themultiplex signal of a subcarrier group with a small decrease inreception level is selected and demodulated on the receiving side, inthis embodiment the same kind of signals are transmitted, and signalsthat have undergone propagation path compensation are combined on thereceiving side. By this means, it is possible to obtain received signalsof significantly improved quality compared with Embodiment 15.

FIG. 28 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 28, in which parts correspondingto those in FIG. 27 are assigned the same codes as in FIG. 27, receivingsystem 1601 of OFDM-CDMA communication apparatus 1600 has a similarconfiguration to that of receiving system 1510, except for having acombining section 1602 instead of selecting section 1511 (FIG. 27).

Based on the reception levels of subcarrier groups G5 and G6 detected bypropagation path compensation circuit 112, and in accordance with acomparison of these reception levels, combining section 1602 performsmaximal-ratio combining of the code division multiplexed signals of thetwo subcarrier groups G5 and G6 for code division multiplexed signalswith a low degree of signal multiplexing input from selecting section611.

According to the above configuration, reception quality can besignificantly improved by transmitting a multiplex signal with a lowdegree of signal multiplexing allocated to a plurality of subcarriergroups, and performing demodulation on the receiving side aftercombining the multiplex signals of these subcarriers.

In the above-described configuration, a case has been described in whichsignals of a plurality of subcarriers with a low degree of signalmultiplexing are subjected to maximal-ratio combining, but the presentinvention is not limited to this, and equal-gain combining, for example,may also be used.

Embodiment 17

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 17 of the present invention is that, when only a known signalis transmitted on specific subcarriers, the subcarriers that transmitthe known signal only are changed according to the cell.

Generally, in OFDM-CDMA communication, a method is used whereby only aknown signal is superimposed on specific subcarriers (generally called“pilot carriers”) These pilot carriers are normally used for residualphase error detection, etc., when demodulation is carried out. In thisembodiment, subcarriers in which pilot carriers are inserted are changedaccording to the cell.

By this means, cell identification characteristics can be furtherimproved compared with Embodiment 9. In other words, to compare thisembodiment with Embodiment 9, in Embodiment 9 signals in which a knownsignal is spread are multiplexed in subcarriers for which the degree ofsignal multiplexing is set low, but in this embodiment subcarriers areformed only by signals in which a known signal is spread.

It is also possible to use a combination of the configuration accordingto this embodiment and the configuration according to Embodiment 9. Inthis case, it is not absolutely necessary for pilot carriers and a knownsignal inserted in other subcarriers for which the degree ofmultiplexing is set low (generally called “pilot channel”) both to beused, and it is also possible, for example, to use a pilot channel foranother purpose (such as for residual phase error detection, forinstance) By so doing, a new effect of enabling the pilot channel to beused for a different purpose is also achieved. Moreover, it is alsopossible to insert only pilot carriers and not to insert a pilotchannel.

FIG. 30 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 30, in which parts correspondingto those in FIG. 19 are assigned the same codes as in FIG. 19,transmitting system 1701 of OFDM-CDMA communication apparatus 1700 has asimilar configuration to that of transmitting system 901 in FIG. 19, butdiffers in that a known signal is input to parallel/serial converter(P/S) 902 without being multiplexed with other signals.

Based on information on the cell to which the far-end station belongs,parallel/serial converter 902 sends signal S5 in which a known signal isspread in an order such that spread signal S5 is allocated tosubcarriers at positions corresponding to the cell to which the far-endstation belongs.

In receiving system 1710, a transmit signal is obtained by havingdespreading processing executed by despreader 1711, and a known signalis obtained by having despreading processing executed by despreader1712. Also, a cell identification signal is obtained based on thesubcarrier positions of the known signal obtained by despreader 1712(these positions can easily be determined by the position at which theknown signal is arrayed after despreading). By this means, the cell towhich this station belongs can easily be identified by receiving system1710.

In FIG. 30, a case in which only a known signal is transmitted byspecific subcarriers (that is, a case in which the degree ofmultiplexing of a known signal is 1) has been described, but the presentinvention is not limited to this, and it is also possible to performtransmission with a known signal multiplexed with other transmit signalsby means of the same subcarriers. The essential point is for thesubcarriers to which the known signal is allocated to be changedaccording to the cell to which the communicating station belongs.

According to the above configuration, by changing the subcarriers towhich a known signal is allocated according to the cell, it is possiblefor the receiving side to easily identify the cell to which it belongsby detecting the positions of subcarriers by which the known signal wastransmitted, even if a signal for cell identification is nottransmitted. As a result, an OFDM-CDMA communication apparatus 1700 canbe implemented that, in addition to achieving the effect obtained inEmbodiment 1, improves transmission efficiency inasmuch as a signal forcell identification is not transmitted.

Embodiment 18

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 18 of the present invention is that the number of cells thatcan be identified can be further increased, compared with Embodiment 17,by changing the kind of known signal of subcarriers that transmit only aknown signal, according to the cell.

When the number of subcarriers used is small, there are not many choicesavailable for subcarriers in which pilot carriers are inserted, andtherefore the number of cells that can be identified is small. Inconsideration of this point, in this embodiment the number of cells thatcan be identified is increased by changing the kind of known signalsuperimposed on pilot carriers according to the cell.

FIG. 31 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 31, in which parts correspondingto those in FIG. 30 are assigned the same codes as in FIG. 30,transmitting system 1801 of OFDM-CDMA communication apparatus 1800 has asimilar configuration to that of transmitting system 1701 in FIG. 30,but differs in having a selecting section 1802 that selects and outputsone of a plurality of known signals 1 through M.

Selecting section 1802 selects and outputs one of a plurality of knownsignals 1 through M, based on cell information. By this means,transmitting system 1801 can form a number of cell identificationinformation items equivalent to the combinations of kinds of knownsignal and subcarrier positions, making this embodiment extremelyeffective when used in an OFDM-CDMA apparatus that has a small number ofsubcarriers relative to the number of cells.

FIG. 31 shows an OFDM-CDMA communication apparatus 1800 that has boththe special feature of changing the kind of known signal of subcarriersthat transmit only a known signal according to the cell, in accordancewith this embodiment, and the special feature of changing thesubcarriers that transmit only a known signal according to the cell, inaccordance with Embodiment 17. However, it is of course also possible tohave only a configuration that changes the kind of known signal ofsubcarriers that transmit only a known signal according to the cell.

Embodiment 19

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 19 of the present invention is that, in addition to providingthe configuration of Embodiment 17 or Embodiment 18, the signal level ofa known signal is made higher than the signal level of other signals. Bythis means, the signal-to-noise ratio of a known signal superimposed onpilot carriers can be increased, enabling cell identificationcharacteristics to be significantly improved.

FIG. 32 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 32, in which parts correspondingto those in FIG. 30 are assigned the same codes as in FIG. 30,transmitting system 1901 of OFDM-CDMA communication apparatus 1900 has asimilar configuration to that of transmitting system 1701 in FIG. 30,but differs in having a multiplier 1902 that multiplies a spread knownsignal by a multiplication coefficient (>1).

Multiplier 1902 raises the transmission level of pilot carriers byperforming amplification weighting on a spread known signal. By thismeans, the receiving side can perform cell identification based on pilotcarriers with a high signal level, enabling the reliability of cellidentification to be improved.

Embodiment 20

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 20 of the present invention is that a bit shifting circuit isprovided instead of the multiplier 1902 in Embodiment 19 (FIG. 32) Bythis means, processing to make the signal level of a spread known signalhigher than the signal level of other subcarriers can be implementedwith a simple configuration.

FIG. 33 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 33, in which parts correspondingto those in FIG. 32 are assigned the same codes as in FIG. 32, intransmitting system 2001 of OFDM-CDMA communication apparatus 2000 aspread known signal is input to a one-bit shifting circuit 2002 thatperforms a one-bit up-shift.

By this means, a signal with twice the signal level of the input signalis output from one-bit shifting circuit 2002. Thus, weighting processingequivalent to that of a multiplier can be performed with a simpleconfiguration comprising one-bit shifting circuit 2002.

In this embodiment, a case has been described in which the transmissionlevel is doubled, but doubling is not a limitation, and any value thatcan be realized with only a bit shifting circuit and adder/subtractercan be set.

According to the above configuration, by using a bit shifting circuit toperform processing that makes the transmission level of subcarriers thattransmit only a known signal higher than the transmission level of othersubcarriers, the same effect as in Embodiment 19 can be achieved with asimple configuration.

Embodiment 21

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 21 of the present invention is that, in addition to providingthe configuration of Embodiment 17, subcarriers that transmit only aknown signal are changed to different subcarriers only at the start of aframe. By this means, frame synchronization characteristics can beimproved on the receiving side since frame synchronization acquisitioncan be performed using both pilot carriers and a pilot channel.

FIG. 34 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 34, in which parts correspondingto those in FIG. 31 are assigned the same codes as in FIG. 31,transmitting system 2101 of OFDM-CDMA communication apparatus 2100 has asimilar configuration to that of transmitting system 1801 in FIG. 31except for the configuration of parallel/serial converter (P/S) 2102.

A signal indicating the start of a frame is input to parallel/serialconverter 2102. Parallel/serial converter 2102 performs spread knownsignal output in an order such that spread known signals 1 through M arechanged to specific subcarriers. In addition, at the start of a frameonly, parallel/serial converter 2102 outputs spread known signal S5 inan order such that signal S5 is allocated to different subcarriers fromthose during other signal periods. By this means, pilot carriers areallocated to other subcarriers only at the start of a frame.

A receiving system for performing frame synchronization based on thesepilot carriers may have a similar configuration to that of receivingsystem 1210 shown in FIG. 22 described in Embodiment 12.

According to the above configuration, frame synchronization detectioncharacteristics can be improved by changing the pilot carrier positionsonly at the start of a frame.

Embodiment 22

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 22 of the present invention is that, as compared withEmbodiment 17, in subcarriers that transmit only a known signal, thekind of known signal is changed only at the start of a frame.

When the number of subcarriers used is small, there are not many choicesavailable for subcarriers in which pilot carriers are inserted, andtherefore when cell identification is performed by means of the pilotcarrier insertion positions, as in Embodiment 17, the number of cellsthat can be identified is also small. In addition, when subcarriers inwhich pilot carriers are inserted are changed only at the start of aframe, as in Embodiment 21, the number of cells that can be identifiedis further reduced.

In consideration of this point, in this embodiment the number of cellsthat can be used is increased by fixing the subcarriers that transmitonly a known signal, and also changing the kind of known signal only atthe start of a frame.

FIG. 35 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 35, in which parts correspondingto those in FIG. 31 are assigned the same codes as in FIG. 31,transmitting system 2201 of OFDM-CDMA communication apparatus 2200 has asimilar configuration to that of transmitting system 1801 of Embodiment18, but differs in that a signal indicating the start of a frame isinput to selecting section 2202 in addition to cell information.

By this means, the operation in transmitting system 2201 is the same asin transmitting system 1801 (FIG. 31) described in Embodiment 18, butdiffers in that, in addition to the operation of above-describedtransmitting system 1801, the kind of known signal is changed only atthe start of a frame.

Frame synchronization acquisition is also possible by combining changingof pilot carrier data at the start of a frame, which is a specialfeature of this embodiment, with changing of the pilot carrier positionsat the start of a frame, which is a special feature of Embodiment 21.

According to the above configuration, by changing the kind of knownsignal only at the start of a frame in subcarriers that transmit only aknown signal, it is possible to have the receiving side identify thecell to which the station it self belongs from among many cells, andalso perform frame synchronization, without adding information for cellidentification or frame synchronization.

Embodiment 23

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 23 of the present invention is that, in addition to providingthe configuration of Embodiments 1 through 22, the signal level islowered for code division multiplexed signals other than code divisionmultiplexed signals for which the degree of multiplexing is set low.That is to say, the signal levels of other code division multiplexedsignals are made relatively lower than the signal levels of codedivision multiplexed signals for which the degree of multiplexing is setlow.

If the signal levels of code division multiplexed signals for which thedegree of multiplexing is set low are made higher, it is possible tosignificantly improve the quality of data for which better quality isrequired than for other data, but when the proportion of subcarriersaccounted for by subcarriers to which code division multiplexed signalsfor which the degree of multiplexing is set low are allocated increases,peak power also increases.

In consideration of this point, in this embodiment, when the proportionof subcarriers for which the degree of multiplexing is set low is large,the signal power of other subcarriers is decreased. By this means, peakpower can be reduced.

FIG. 36 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 36, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 2301 of OFDM-CDMA communication apparatus 2300 has asimilar configuration to that of transmitting system 101 in FIG. 6, butdiffers in having multipliers 2302, . . . , 2303, 2304 that executeweighting processing on code division multiplexed signals S1, . . . ,S4, S5.

The value selected as the multiplication coefficient (coefficient 2) ofmultipliers 2302, . . . , 2303 provided for code division multiplexedsignals with a high degree of signal multiplexing S1, . . . , S4 issmaller than the value of the multiplication coefficient (coefficient 1)of multiplier 2304 provided for code division multiplexed signal S5.That is to say, the relationship “coefficient 2<coefficient 1” holdstrue.

In this embodiment, the signal level of code division multiplexed signalS5 for which the degree of multiplexing is set low is decreased by alsoproviding a multiplier 2304 for code division multiplexed signal S5 forwhich the degree of multiplexing is set low, and selecting a valuesmaller than “1” for coefficient 1.

According to the above configuration, peak power—which is a problem inOFDM-CDMA transmission—is reduced satisfactorily by lowering signallevels taking into consideration the proportion of all subcarriersaccounted for by subcarriers for which the degree of signal multiplexingis set low.

Embodiment 24

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 24 of the present invention is that, as compared withEmbodiment 23, the signal power of code division multiplexed signalsallocated to subcarriers is controlled by means of bit shifting circuitsinstead of multipliers. By this means, the same effect as in Embodiment23 can be achieved with a significantly simpler configuration.

FIG. 37 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 37, in which parts correspondingto those in FIG. 36 are assigned the same codes as in FIG. 36,transmitting system 2401 of OFDM-CDMA communication apparatus 2400 has asimilar configuration to that of transmitting system 2301 in FIG. 36,except for being provided with one-bit shifting circuits 2402, . . . ,2403, 2404 instead of multipliers 2302, . . . , 2303, 2304.

One-bit shifting circuits 2402, . . . , 2403 that perform a one-bitdown-shift are provided for code division multiplexed signals S1, . . ., S4 with a high degree of multiplexing, while one-bit shifting circuit2401 that performs a one-bit up-shift is provided for code divisionmultiplexed signal S5 with a low degree of multiplexing. By this means,one-bit shifting circuits 2402, . . . , 2403 halve the signal level ofcode division multiplexed signals S1, . . . , S4 with a high degree ofsignal multiplexing, and one-bit shifting circuit 2404 doubles thesignal level of code division multiplexed signal S5 with a low degree ofsignal multiplexing.

In this embodiment, a case has been described in which signal levels aredoubled or halved by providing one-bit shifting circuits, butamplification levels are not limited to these, and any value that can berealized with only a bit shifting circuit and adder/subtracter can beset.

According to the above configuration, peak power—which is a problem inOFDM-CDMA transmission—is reduced satisfactorily and with a simpleconfiguration by performing processing to increase or decrease signallevels using bit shifting circuits, taking into consideration theproportion of all subcarriers accounted for by subcarriers for which thedegree of signal multiplexing is set low.

Embodiment 25

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 25 of the present invention is that, in addition to theprovisions of Embodiment 1, the signal level is made variable for eachsubcarrier according to the channel quality.

With regard to the transmission output of each subcarrier, quality isnaturally better the higher the transmission output, but a drawback isthat, conversely, when transmission power increases, peak power andpower consumption also increase. In this embodiment this point is takeninto consideration, and error rate characteristics and peak power aremade compatible by adaptively changing the signal level of eachsubcarrier according to the channel quality.

In the case of this embodiment, received field level information(generally called “RSSI” (Received Signal Strength Indicator)) is usedas an indicator for changing signal levels. By this means, it ispossible to improve error rate characteristics and also suppress anundesired increase in peak power by increasing the signal levelproportionally the lower the received field level.

FIG. 38 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 38, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 2501 of OFDM-CDMA communication apparatus 2500 has asimilar configuration to that of transmitting system 101 in FIG. 6, butdiffers in having a selecting section 2502 that selects eithercoefficient 1 or coefficient 2 according to RSSI, and a multiplier 2503that changes the signal level of code division multiplexed signal S5with a low degree of signal multiplexing by multiplying code divisionmultiplexed signal S5 by the selected coefficient.

The relationship between coefficient 1 and coefficient 2 is “coefficient1<coefficient 2”. Selecting section 2502 selects and outputs smallercoefficient 1 when RSSI is greater than a predetermined threshold value,and selects larger coefficient 2 when RSSI is less than or equal to thethreshold value. As a result, the signal level of code divisionmultiplexed signal S5 with a low degree of multiplexing is increased ina propagation environment in which the received field level is low, andconversely, is decreased in a propagation environment in which thereceived field level is high. By this means, it is possible to for anundesired increase in peak power to be suppressed and error ratecharacteristics to be improved in transmitting system 2501.

In the sample configuration shown in FIG. 38, a case is illustrated inwhich only the signal level of subcarriers with a low degree of signalmultiplexing is changed adaptively, but the present invention is notlimited to this, and it is also possible to change adaptively the signallevel of subcarriers with a high degree of signal multiplexing. Also, acase has been described in which there are two choices of signal level,but the present invention is not limited to this, and any selections arepossible. Moreover, a case has been described in which RSSI is used asan indicator for changing signal levels, but a parameter other thanRSSI, such as delay distribution, for example, can also be used, as longas that parameter serves as an indicator of channel quality.

According to the above configuration, by adaptively changing the signallevel of a code division multiplexed signal allocated to subcarriersaccording to channel quality, in addition to offering the provisions ofEmbodiment 1, it is possible to suppress an undesired increase in peakpower and improve error rate characteristics, in addition to achievingthe effect of Embodiment 1.

Embodiment 26

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 26 of the present invention is that, as compared withEmbodiment 1, subcarriers for which the degree of signal multiplexing isset low, and other subcarriers, are transmitted from different antennas.

By this means, the transmission power of the amplifier in eachtransmitting section can be reduced, enabling peak power to be reduced.Also, as a result of this, the transmission power of subcarriers forwhich the degree of signal multiplexing is set low can be increased,making it possible to further improve the quality of data for whichbetter quality is required than for other data, allocated to subcarriersfor which the degree of signal multiplexing is reduced.

FIG. 39 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 39, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 2601 of OFDM-CDMA communication apparatus 2600 has asimilar configuration to that of transmitting system 101 in FIG. 6, butdiffers in having two antennas AN1 and AN2, and two transmissionprocessing sections corresponding thereto.

Transmitting system 2601 transmits code division multiplexed signals S1through S4 with a high degree of signal multiplexing from antenna AN1via parallel/serial converter 102, inverse fast Fourier transformcircuit 103-1, and radio transmitting section 104-1. On the other hand,transmitting system 2601 transmits code division multiplexed signal S5with a low degree of multiplexing from antenna AN2 via inverse fastFourier transform circuit 103-2 and radio transmitting section 104-2.

According to the above configuration, by transmitting subcarriers forwhich the degree of signal multiplexing is set low, and othersubcarriers, from different antennas, the signal level of subcarriersfor which the degree of signal multiplexing is set low can be increased,and as a result, important information can be transmitted withsignificantly higher quality.

Embodiment 27

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 27 of the present invention is that, in addition to theprovisions of Embodiment 1, the degree of signal multiplexing ofsubcarriers for which the degree of signal multiplexing is set low ismade variable.

The higher the degree of signal multiplexing, the higher, naturally, istransmission efficiency, but conversely, the poorer are error ratecharacteristics. Thus, the fact that the optimal value of degree ofsignal multiplexing differs according to channel quality was considered,and it was thought that an improvement in error rate characteristics andan improvement in transmission efficiency could be made compatible to asignificantly greater extent by changing the degree of signalmultiplexing according to a parameter such as RSSI, for example.

FIG. 40 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 40, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 2701 of OFDM-CDMA communication apparatus 2700 has asimilar configuration to that of transmitting system 101 in FIG. 6, butdiffers in that a selecting section 2702 that selects signals to bemultiplexed is provided in the processing system that forms multiplexedsignals of subcarriers with a low degree of signal multiplexing.

Selecting section 2702 changes the number of spread transmit signals tobe output to next-stage adder B5 according to the size of RSSI.Specifically, the larger RSSI, the better the channel quality can beconsidered to be, and therefore when RSSI is large, transmissionefficiency is raised by increasing the number of spread transmit signalsto be output. Conversely, the smaller RSSI, the poorer the channelquality can be considered to be, and therefore when RSSI is small,degradation of error rate characteristics is prevented by decreasing thenumber of spread transmit signals to be output.

In this embodiment, a case has been described in which RSSI is used asan indicator for selecting the degree of signal multiplexing ofsubcarriers for which the degree of signal multiplexing is set low, butit is also possible to use a parameter other than RSSI, such as delaydistribution, for example, as long as that parameter serves as anindicator of channel quality.

According to the above configuration, by adaptively changing the degreeof signal multiplexing of subcarriers for which the degree of signalmultiplexing is set low according to channel quality, in addition tooffering the provisions of Embodiment 1, it is possible to make animprovement in error rate characteristics compatible with an improvementin transmission efficiency to a significantly greater extent, inaddition to achieving the effect of Embodiment 1.

Embodiment 28

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 28 of the present invention is that, in addition to provisionof the configuration according to Embodiment 12, the accuracy of framesynchronization detection is improved by making the degree ofmultiplexing smaller for a symbol at the start of a frame than for othersymbols, for subcarriers for which the degree of signal multiplexing isset low.

As the degree of signal multiplexing increases, the accuracy of framesynchronization detection naturally falls. However, the lower the degreeof signal multiplexing is made, the greater is the fall in transmissionefficiency. In consideration of this point, in this embodiment theaccuracy of frame synchronization detection is improved without greatlydecreasing transmission efficiency by reducing the degree of signalmultiplexing only at the start of a frame.

FIG. 41 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 41, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 2801 of OFDM-CDMA communication apparatus 2800 has asimilar configuration to that of transmitting system 101 in FIG. 6, butdiffers in that a known signal is input to spreader A(x+1) via an on/offswitch (ON/OFF) 2803, and that a selecting section 2802 is provided thatselects transmit signals.

On/off switch 2803 outputs a known signal only at the start of a frame.At the start of a frame, selecting section 2802 does not output all theinput plurality of transmit signals, but outputs only a predeterminednumber thereof. It is here assumed that the number of signalsmultiplexed by adder B5 is smaller than the number of signalsmultiplexed by other adders B1 through B4, in the same way as in otherabove-described embodiments.

By this means, in transmitting system 2801, a known signal istransmitted only at the start of a frame by subcarriers for which thedegree of signal multiplexing is set low, and also, the degree of signalmultiplexing of those subcarriers is significantly reduced at the startof that frame. As a result, frame synchronization can be performed witha high degree of accuracy on the receiving side, where framesynchronization is performed at the timing at which a known signal isdetected.

According to the above configuration, by reducing the number of signalsmultiplexed with a known signal at the start of a frame, in addition tomultiplexing signals in which that known signal is spread only at thestart of that frame, as described in Embodiment 12, it is possible toimprove the accuracy of frame synchronization detection by the receivingside while suppressing a fall in transmission efficiency, in addition toachieving the effect of Embodiment 12.

Embodiment 29

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 29 of the present invention is that a user for which thereare a large number of retransmissions is allocated preferentially tosubcarriers for which the degree of signal multiplexing is set low. Bythis means, the number of retransmissions can be reduced, making itpossible to prevent a drop in throughput and transmission delay time.

As the number of retransmissions increases, the drop in throughput andtransmission delay time also increases, and therefore it is necessary tominimize the number of retransmissions. In consideration of this point,in this embodiment a retransmission signal is allocated preferentiallyto subcarriers with a low degree of signal multiplexing. By this means,retransmission signal quality can be improved, and thus the number ofretransmissions can be reduced.

In this embodiment, also, note is taken of the fact that when the numberof users is large and there are many retransmission bursts, it is notalways possible to allocate a retransmission burst to subcarriers forwhich the degree of signal multiplexing is set low. Taking such a caseinto consideration, a user with more retransmissions than other users isallocated preferentially to subcarriers for which the degree of signalmultiplexing is set low. By this means, a further increase in the numberof retransmissions can be prevented.

FIG. 42 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 42, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 2901 of OFDM-CDMA communication apparatus 2900 has asimilar configuration to that of transmitting system 101 in FIG. 6, butdiffers in that a selecting section 2902 is provided in the processingsystem that forms code division multiplexed signal S5 with a low degreeof signal multiplexing, and retransmission signals 1 through m are inputto this selecting section 2902 in addition to transmit signals 4 k+1through n.

Based on a signal indicating the number of retransmissions from acontrol section (not shown), selecting section 2902 selects fromretransmission signals 1 through m a signal for which the number ofretransmissions is large (for example, a user performing at least athird retransmission) in preference to transmit signals 4 k+1 through n.By this means, a user for which the number of retransmissions is largeis allocated preferentially to subcarriers for which the degree ofsignal multiplexing is set low.

According to the above configuration, by preferentially allocating auser for which there are a large number of retransmissions tosubcarriers for which the degree of signal multiplexing is set low, thenumber of retransmissions can be reduced, and a drop in throughput andtransmission delay time can be prevented.

Embodiment 30

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 30 of the present invention is that a null signal istransmitted by subcarriers on either side of a subcarrier group forwhich the degree of multiplexing is set low. By this means, it ispossible to prevent degradation of error rate characteristics when thespeed of movement is high.

When the speed of movement is high, interference between subcarriersincreases. The degree of multiplexing of subcarriers on either side of asubcarrier group for which the degree of multiplexing is set low is sethigher than for the aforementioned subcarrier group, and thereforeinterference between subcarriers increases and there is greaterdegradation of error rate characteristics. Interference betweensubcarriers is especially great when the subcarriers on either side of asubcarrier group for which the degree of multiplexing is set low use amodulation method with a high modulation M-ary value, such as 16QAM(Quadrature Amplitude Modulation) or 64QAM.

In this embodiment this point is taken into consideration, and bytransmitting a null signal in subcarriers #m and #2 m+1 on either sideof subcarrier group #m+1 through #2 m for which the degree ofmultiplexing is set low, as shown in FIG. 43, reception by subcarriergroup #m+1 through #2 m for which the degree of multiplexing is set lowof interference from adjacent subcarriers #m and #2 m+1 is prevented,and degradation of the error rate characteristics of that subcarriergroup by which important information is transmitted is also prevented.

FIG. 44 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 44, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 3001 of OFDM-CDMA communication apparatus 3000 has asimilar configuration to that of transmitting system 101 in FIG. 6, butdiffers in that a null signal is input to parallel/serial converter 102.

Parallel/serial converter 102 outputs code division multiplexed signalS5 and null signals in an arrangement whereby a null signal istransmitted by subcarriers on either side of a subcarrier group forwhich the degree of multiplexing is set low.

According to the above configuration, by transmitting a null signal insubcarriers on either side of a subcarrier group for which the degree ofmultiplexing is set low, the error rate characteristics of thatsubcarrier group can be significantly improved.

Embodiment 31

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 30 of the present invention is that subcarriers for which thedegree of multiplexing is set low are located at intervals of aplurality of subcarriers, and also a null signal is transmitted bysubcarriers between subcarriers for which the degree of multiplexing isset low. By this means, it is possible to prevent degradation of errorrate characteristics to a greater extent that in Embodiment 30 when thespeed of movement is high.

When the speed of movement is extremely high, interference also occursbetween subcarriers for which the degree of multiplexing is set low. Inconsideration of this point, in this embodiment subcarriers for whichthe degree of multiplexing is set low are located at intervals of aplurality of subcarriers, and a null signal is transmitted bysubcarriers between subcarriers for which the degree of multiplexing isset low. By this means, it is possible to decrease interference betweensubcarriers for which the degree of multiplexing is set low. As aresult, in comparison with above-described Embodiment 30, it is possibleto further prevent degradation of error rate characteristics when thespeed of movement is high.

FIG. 45 shows the configuration of an OFDM-CDMA signal according to thisembodiment. As shown in FIG. 45, subcarriers #m+1 and #2 m for which thedegree of multiplexing is set low are arranged non-adjacently, and anull signal is transmitted by subcarriers #m+2, . . . , #2 m−1 betweenthese subcarriers #m+1 and #2 m.

FIG. 46 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 46, in which parts correspondingto those in FIG. 44 are assigned the same codes as in FIG. 44,transmitting system 3101 of OFDM-CDMA communication apparatus 3100 has asimilar configuration to that of transmitting system 3001 in FIG. 44,but differs in that a null signal is input to adder B5 that forms codedivision multiplexed signal S5 with a low degree of signal multiplexing.

Adder B5 forms code division multiplexed signal S5 by input signals sothat a null signal is input between spread transmit signals 4 k+1, . . ., n. As in Embodiment 30, parallel/serial converter 102 outputs codedivision multiplexed signal S5 and null signals in an arrangementwhereby a null signal is transmitted by subcarriers on either side of asubcarrier for which the degree of multiplexing is set low.

According to the above configuration, by locating subcarriers for whichthe degree of multiplexing is set low at intervals of a plurality ofsubcarriers, and also transmitting a null signal by subcarriers betweensubcarriers for which the degree of multiplexing is set low, it ispossible to prevent degradation of error rate characteristics to asignificantly greater extent that in Embodiment 30 when the speed ofmovement is high.

Embodiment 32

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 32 of the present invention is that amplitude limiting isperformed independently for subcarriers for which the degree ofmultiplexing is set low and for other subcarriers. For example,amplitude limiting may be performed for subcarriers with a high degreeof multiplexing, while amplitude limiting is not performed forsubcarriers for which the degree of multiplexing is set low. By thismeans, it is possible to make an improvement in error ratecharacteristics compatible with a reduction of peak power forsubcarriers for which the degree of multiplexing is set low, to agreater extent than in other embodiments described above.

Peak power is a problem when transmitting an OFDM-CDMA signal.Therefore, processing is generally carried out to reduce peak power byperforming amplitude limiting. However, a drawback of amplitude limitingis that it results in degradation of error rate characteristics.

In consideration of this point, in this embodiment the error ratecharacteristics of subcarriers for which the degree of multiplexing isset low are significantly improved by not performing amplitude limitingon subcarriers for which the degree of multiplexing is set low.Amplitude limiting is performed for other subcarriers.

As the number of subcarriers for which the degree of multiplexing is setlow represents a small proportion of the total number of subcarriers,and a low degree of multiplexing implies proportionally low peak power,the effect of reducing peak power is scarcely lessened by not performingamplitude limiting on subcarriers for which the degree of multiplexingis set low.

Thus, by not performing amplitude limiting on subcarriers for which thedegree of multiplexing is set low, compatibility between error ratecharacteristics and peak power reduction for subcarriers for which thedegree of multiplexing is set low can be achieved to a significantlygreater extent than in other embodiments described above.

FIG. 47 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 47, in which parts correspondingto those in FIG. 46 are assigned the same codes as in FIG. 46,transmitting system 3201 of OFDM-CDMA communication apparatus 3200 has asimilar configuration to that of transmitting system 3101 in FIG. 46,but differs in being provided with amplitude limiting sections C1through C4 that limit the signal amplitude of code division multiplexedsignals S1 through S4 with a high degree of signal multiplexing.Amplitude limiting is not performed on code division multiplexed signalS5 with a low degree of signal multiplexing.

In FIG. 47, a case is illustrated in which amplitude limiting is notperformed on code division multiplexed signal S5 for which the degree ofmultiplexing is set low, but the present invention is not limited tothis, and virtually the same effect as in the case illustrated in FIG.47 can be achieved, for example, by making the amplitude limiting forcode division multiplexed signal S5 allocated to subcarriers for whichthe degree of multiplexing is set low less stringent than the amplitudelimiting applied to code division multiplexed signals S1 through S4allocated to subcarriers for which the degree of multiplexing is madehigh; the essential point being to perform amplitude limitingindependently for subcarriers for which the degree of multiplexing isset low and for other subcarriers.

According to the above configuration, by performing amplitude limitingindependently for subcarriers for which the degree of multiplexing isset low and for other subcarriers, the adverse effects of peak power arekept to a minimum, and the error rate characteristics of subcarriers forwhich the degree of multiplexing is set low can be significantlyimproved.

Embodiment 33

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 33 of the present invention is that, in contrast to theabove-described embodiments, the number of a preamble for propagationpath estimation is set independently for each subcarrier. In the case ofthis embodiment, a large number of propagation path estimation preambleis inserted for subcarriers for which the degree of multiplexing is setlow than for other subcarriers. By this means, the error ratecharacteristics of transmit signals superimposed on subcarriers forwhich the degree of multiplexing is set low can be significantlyimproved.

The more numerous the propagation path estimation preamble inserted, thebetter are the error rate characteristics. However, as the number of thepropagation path estimation preamble inserted increases, transmissionefficiency decreases, as correspondingly fewer other signals can betransmitted. In the case of channel conditions in which adequate errorrate characteristics can be obtained even if a low propagation pathestimation preamble is set, in particular, transmission efficiency willonly fall even if a large number of propagation path estimation preambleis set, and the effect will be small.

In consideration of these points, in this embodiment a large number ofpropagation path estimation preamble is set only for subcarriers forwhich a low degree of multiplexing is set. As a result, compatibilitycan be achieved to a significant extent between an improvement in theerror rate and transmission efficiency.

FIG. 48 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 48, in which parts correspondingto those in FIG. 39 are assigned the same codes as in FIG. 39, intransmitting system 3301 of OFDM-CDMA communication apparatus 3300 onekind of pilot symbol 1 is input to parallel/serial converter 102-1, intowhich code division multiplexed signals S1 through S4 with a high degreeof signal multiplexing are input. On the other hand, two kinds of pilotsymbols 1 and 2 are input to parallel/serial converter 102-2, into whichcode division multiplexed signal S5 with a low degree of signalmultiplexing is input.

Parallel/serial converter 102-1 rearranges code division multiplexedsignals S1 through S4 into a predetermined order, inserts pilot symbol 1at a predetermined position, and outputs serial signal S6-1. Meanwhile,parallel/serial converter 102-2 inserts pilot symbol 1 and pilot symbol2 at predetermined positions in the code division multiplexed signal,and outputs serial signal S6-2. Serial signals S6-1 and S6-2 are thentransmitted from antenna AN1 via IFFT 103-1 and RF 104-1, and fromantenna AN2 via IFFT 103-2 and RF 104-2, respectively.

As a result, as shown in FIG. 49, subcarriers #1 through #m, #2 m+1through #3 m, #3 m+1 through #4 m, and #4 m+1 through #5 m with a highdegree of signal multiplexing, in which only pilot symbol 1 is insertedas a propagation path estimation preamble, are transmitted from antennaAN1, while subcarriers #m+1 through #2 m with a low degree of signalmultiplexing, in which pilot symbol 1 and pilot symbol 2 are inserted asa propagation path estimation preamble, are transmitted from antennaAN2.

In this embodiment, a case has been described in which the number of apropagation path estimation preamble is set as one symbol or two symbolsaccording to the subcarrier, but the present invention is not limited tothis, and any number can be set for the propagation path estimationpreamble. It is also possible for the number of the propagation pathestimation preamble not to be set as a fixed value, but to be variedadaptively according to channel quality, etc.

According to the above configuration, by setting a large number ofpropagation path estimation preamble for subcarriers for which thedegree of multiplexing is set low than for other subcarriers, it ispossible to significantly improve the error rate of a signal transmittedby subcarriers for which the degree of multiplexing is set low, whilesuppressing a drop in transmission efficiency.

Embodiment 34

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 34 of the present invention is that a code-divided signal,and that code-divided signal shifted by one or more chips, aremultiplexed. This multiplexed signal is then transmitted assigned to aplurality of subcarriers. By this means, error rate characteristics canbe improved when subcarriers are subjected to fluctuationsindependently.

In a multipath environment, each subcarrier is subjected to amplitudefluctuations independently, and therefore amplitude deviation occursbetween chips, and orthogonality between spreading codes is disrupted.Thus, depending on the propagation environment, disruption oforthogonality may be severe. For example, disruption of orthogonalitywill be severe in the event of either +1 code or −1 code bias insubcarriers whose reception level has declined.

Focusing on this point, a code division multiplexed signal ismultiplexed with a code division multiplexed signal representing thefirst-mentioned code division multiplexed signal shifted by one or morechips before being transmitted. The receiving side is provided with afirst despreading section that despreads the code division multiplexedsignal that is not shifted, and a second despreading section thatdespreads the code division multiplexed signal that is shifted. Thesetwo despreading sections can have a configuration whereby a correlatorcoefficient is shifted by the number of chips by which the spreadingcode was shifted on the transmitting side.

Since the probability of disruption of orthogonality being severe forboth the unshifted code division signal and the shifted code divisionsignal is low, a received signal with good error rate characteristicscan be obtained by selecting the despread value of one or other of thesecode division multiplexed signals.

In this embodiment, a code division multiplexed signal, and that codedivision multiplexed signal shifted by one or more chips, aremultiplexed for subcarriers for which the degree of signal multiplexingis set lower than for other subcarriers. By this means, the error ratecharacteristics of subcarriers for which the degree of multiplexing isset low can be significantly improved.

FIG. 50 shows an example in which a code division multiplexed signal,and that code division multiplexed signal shifted by (subcarrier/2)chips, are multiplexed in subcarriers #m+1 through #2 m for which thedegree of signal multiplexing is set lower than for other subcarriers.As can be seen from FIG. 50, in the (m+1)'th subcarrier, for example,the first chip of plurality of transmit signals 4 k+1 through n in timeT is code division multiplexed, and a code division multiplexed signalrepresenting the first chip of plurality of transmit signals 4 k+1through n in time T shifted by m/2 chips is multiplexed. Similarly, inthe 2 m'th subcarrier, the m'th chip of plurality of transmit signals 4k+1 through n in time T is code division multiplexed, and a codedivision multiplexed signal representing the m'th chip of plurality oftransmit signals 4 k+1 through n in time T shifted by m/2 chips ismultiplexed.

FIG. 51 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In FIG. 51, in which parts correspondingto those in FIG. 6 are assigned the same codes as in FIG. 6,transmitting system 3401 of OFDM-CDMA communication apparatus 3400 isprovided with a serial/parallel converter (S/P) 3402 and parallel/serialconverter (P/S) 3403 as an example of a section for shifting codedivision multiplexed signal S5 with a low degree of signal multiplexingby one or more chips. Transmitting system 3401 is also provided with anadder 3405 as a multiplexing section that multiplexes original codedivision multiplexed signal S5 with the code division multiplexed signalshifted by one or more chips.

Code division multiplexed signal S5 is converted to parallel form byserial/parallel converter 3402, and the spread signal is shifted by(subcarrier/2) chips by means of signal rearrangement performed byparallel/serial converter 3403.

Next, receiving system 3410 of OFDM-CDMA communication apparatus 3400will be described. In OFDM-CDMA communication apparatus 3400, anOFDM-CDMA signal transmitted from an OFDM-CDMA communication apparatuswith a similar configuration is input to fast Fourier transform circuit(FFT) 111 via antenna AN and radio receiving section (RF) 114 thatperforms radio reception processing such as analog/digital conversion.FFT 111 executes fast Fourier transform processing on the input signal.By this means, a code division multiplexed signal superimposed on aplurality of subcarriers is obtained.

Propagation path compensation circuit 112 compensates for phasefluctuations, etc., occurring in the propagation path, based on a knownsignal such as a propagation path estimation preamble included in thesignal. After propagation path compensation, the signal is despread bydespreader 113, and the received signal for that station is extractedfrom the plurality of transmit signals.

Receiving system 3410 is also provided with a first despreading section3411 that despreads the code division multiplexed signal from the signalin which a code division multiplexed signal, and that code divisionmultiplexed signal shifted by one or more chips, are multiplexed, andobtains a received signal, and a second despreading section 3412 thatdespreads the signal shifted by one or more chips, and obtains areceived signal.

That is to say, first despreading section 3411 performs despreadingprocessing using the same spreading code as spreading sections A(4 k+1). . . An of transmitting system 3401, while second despreading section3412 performs despreading processing using a spreading code shifted by(subcarrier/2) with respect to spreading sections A(4 k+1) . . . An.

Of the two despreading results, selecting section 3413 selects andoutputs the one with the larger correlation level. By this means, thecode division multiplexed signal with the lesser disruption oforthogonality can be extracted selectively. The selection method is notlimited to correlation level, and selection based on the well-knownphase likelihood technique or the like may also be used, for example.

According to the above configuration, error rate characteristics can beimproved when subcarriers are subjected to fluctuations independently,by multiplexing a code division multiplexed signal, and that codedivision multiplexed signal shifted by one or more chips, andtransmitting this multiplexed code-division signal assigned to aplurality of subcarriers.

In this embodiment, a case has been described in which a code-dividedsignal, and that code-divided signal shifted by one or more chips, aremultiplexed in subcarriers for which the degree of signal multiplexingis set lower than for other subcarriers, but the present invention isnot limited to this. For example, the error rate of subcarriers withpoor channel quality can be improved by multiplexing a code-dividedsignal, and that code-divided signal shifted by one or more chips, insubcarriers with poor channel quality. Moreover, it is also possible tomultiplex a code-divided signal, and that code division multiplexedsignal shifted by one or more chips, in all subcarriers, and performmulticarrier transmission.

Also, in this embodiment a case has been described in which shifting by(subcarrier/2) chips is performed, but this is not a limitation, and ashift of any number of chips may be set. Moreover, the degree ofmultiplexing is not limited to two signals, and any degree ofmultiplexing may be set. For example, it is possible to multiplex acode-division signal that is not shifted, the code-division signalshifted by (subcarrier/3) chips, and the code-division signal shifted by2×(subcarrier/3) chips.

Embodiment 35

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 35 of the present invention is that the processing thatmultiplexes a code-division signal shifted by one or more chips inEmbodiment 34 is performed only for a specific transmit signal. By thismeans, error rate characteristics can be improved when the degree ofsignal multiplexing is comparatively high.

Here, a spreading code and a signal with that spreading code shifted arenot necessarily in an orthogonal relationship (having a mutualcorrelation of 0). Therefore, as the degree of signal multiplexingincreases, when code division signals shifted by one or more chips aremultiplexed, it may happen that the interference component increasesand, conversely, error rate characteristics degrade.

In consideration of this point, in this embodiment multiplexing of acode division signal shifted by one or more chips is restricted to atransmit signal to a specific user. A user far from a base station, or auser with a low reception level, for example, may be selected as thisspecific user. By this means, it is possible to improve the error ratecharacteristics of a transmit signal to a specific user withoutdegrading error rate characteristics when transmit signals are viewed asa whole.

FIG. 52 shows the configuration of an OFDM-CDMA communication apparatusaccording to this embodiment. In transmitting system 3501 of OFDM-CDMAcommunication apparatus 3500, a code division signal is shifted by oneor more chips by serial/parallel converter (S/P) 3502 andparallel/serial converter (P/S) 3503 for transmit signal n only, and thepre-shift code division signal and post-shift code division signal aremultiplexed by adder 3504.

Receiving system 3510 is also provided with a first despreading section3511 that despreads the code-divided signal from the signal in which acode-divided signal, and that code-divided signal shifted by one or morechips, are multiplexed, and obtains a received signal, and a seconddespreading section 3512 that despreads the signal shifted by one ormore chips, and obtains a received signal.

That is to say, first despreading section 3511 performs despreadingprocessing using the same spreading code as spreading section An oftransmitting system 3501, while second despreading section 3512 performsdespreading processing using a spreading code shifted by a predeterminednumber of chips (for example, subcarrier/2) with respect to spreadingsection An.

Of the two despreading results, selecting section 3513 selects the onewith the larger correlation level. By this means, the code divisionmultiplexed signal with the lesser disruption of orthogonality can beextracted selectively. The selection method is not limited tocorrelation level, and selection based on the well-known phaselikelihood technique or the like may also be used, for example.

According to the above configuration, by performing processing thatmultiplexes a code-division signal shifted by one or more chips only fora specific transmit signal, it is possible to improve the error ratecharacteristics of a specific transmit signal while suppressingdegradation of error rate characteristics when transmit signals areviewed as a whole.

In this embodiment, a case has been described in which processing thatmultiplexes a code-division signal shifted by one or more chips isperformed only for one specific transmit signal n, but this is not alimitation, and any setting may be made. It is also possible to set ahigher degree of multiplexing for a code division signal shifted by oneor more chips for a specific transmit signal than for other transmitsignals. For example, it is possible to multiplex four code divisionsignals with different numbers of shift chips for a specific transmitsignal, and to multiplex two code division signals with differentnumbers of shift chips for other transmit signals.

Embodiment 36

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 36 of the present invention is that a received signal isobtained by combining a signal in which a code division multiplexedsignal is despread, and a signal in which a code division multiplexedsignal shifted by one or more chips is despread, in reception inEmbodiments 34 and 35. By this means, error rate characteristics can beimproved to a greater extent than in Embodiments 34 and 35.

That is to say, in Embodiments 34 and 35 a case has been described inwhich selection diversity is obtained by providing a selecting section3413 (FIG. 51) or selecting section 3513 (FIG. 52) in the receivingsystem, but in this embodiment, a received signal is obtained byperforming combining diversity. By this means, a received signal withimproved error rate characteristics can be obtained, since combiningdiversity offers a 1 dB to 1.5 dB error rate characteristics improvementcompared with selection diversity.

FIG. 53 shows the configuration of the receiving system of an OFDM-CDMAcommunication apparatus according to this embodiment. In FIG. 53, inwhich parts corresponding to those in FIG. 52 are assigned the samecodes as in FIG. 52, receiving system 3601 of OFDM-CDMA communicationapparatus 3600 is provided with a first despreading section 3602 thatdespreads a code division multiplexed signal and obtains a receivedsignal, and a second despreading section 3603 that despreads a signalshifted by one or more chips and obtains a received signal.

In addition, receiving system 3601 is provided with a combining section3604 that combines correlation output from the two despreading sections3602 and 3603. Any combining method, such as equal-gain combining ormaximal-ratio combining, can be used by combining section 3604.

According to the above configuration, by combining a signal in which acode division multiplexed signal is despread and a signal in which acode division multiplexed signal shifted by one or more chips isdespread, it is possible to obtain a received signal that has bettererror rate characteristics than in Embodiments 34 and 35.

Embodiment 37

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 37 of the present invention is that a code division signalshifted by one or more chips and the original code division signal aremultiplexed only for a known signal. By this means, it is possible toimprove the error rate characteristics of a known signal withoutdegrading the error rate characteristics of other transmit signals codedivision multiplexed with the known signal. The known signal may be usedfor cell identification as described in Embodiment 9, for example, orfor propagation path estimation, or for various other purposes. In thisembodiment, by performing multiplexing of a code division signal shiftedby one or more chips with the original code division signal for a knownsignal used for various purposes in this way, it is possible to improvethe error rate characteristics of the known signal, and to improve cellidentification accuracy and received signal quality.

Here, as also explained in Embodiment 35, a spreading code and a signalwith that spreading code shifted are not necessarily in an orthogonalrelationship (having a mutual correlation of 0). Therefore, as thedegree of signal multiplexing increases, when code division signalsshifted by one or more chips are multiplexed, it may happen that theinterference component increases and, conversely, error ratecharacteristics degrade.

In consideration of this point, in this embodiment multiplexing of acode division signal shifted by one or more chips is restricted to aknown signal (this processing is not performed on transmit signals 4 k+1. . . n). By this means, it is possible to improve the error ratecharacteristics of a known signal without degrading error ratecharacteristics when transmit signals are viewed as a whole. As aresult, when, for example, a known signal is used for cellidentification, the accuracy of cell identification can be improved.

FIG. 54 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.54, in which parts corresponding to those in FIG. 52 are assigned thesame codes as in FIG. 52, in transmitting system 3701 of OFDM-CDMAcommunication apparatus 3700, a code division signal is shifted by oneor more chips by serial/parallel converter (S/P) 3702 andparallel/serial converter (P/S) 3703 for a known signal only, and thepre-shift code division signal and post-shift code division signal aremultiplexed by adder 3704.

According to the above configuration, by multiplexing a code divisionsignal shifted by one or more chips and the original code divisionsignal only for a known signal, it is possible to improve the error ratecharacteristics of a known signal without degrading the error ratecharacteristics of other transmit signals 4 k+1 . . . n code divisionmultiplexed with the known signal.

In this embodiment, a case has been described in which a code divisionsignal shifted by one or more chips and the original code divisionsignal are multiplexed only for a known signal, but it is also possibleto set a higher degree of multiplexing for a code division signalshifted by one or more chips than for other transmit signals 4 k+1 . . .n for a known signal. For example, it is possible to multiplex four codedivision signals with different numbers of shift chips for a knownsignal, and to multiplex two code division signals with differentnumbers of shift chips for other transmit signals 4 k+1 . . . n.

Embodiment 38

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 38 of the present invention is that a code division signalshifted by one or more chips and the original code division signal aremultiplexed only at the start of a frame. By this means, it is possiblefor frame synchronization detection processing to be performed on thereceiving side without increasing the number of spreading codes orincreasing the number of known signals.

As the number of spreading codes is limited, the kinds of known signalsto be inserted must be minimized. In consideration of this point, inthis embodiment a code division signal shifted by one or more chips andthe original code division signal are multiplexed only at the start of aframe. This enables frame synchronization detection to be performed onthe receiving side based on the number of correlation peaks.

Specifically, as a code division signal shifted by one or more chips andthe original code division signal are multiplexed only at the start of aframe, a plurality of peaks appear only at the start of a frame in asignal that has undergone despreading. Frame synchronization detectioncan be performed on the receiving side by detecting the timing at whichthis plurality of peaks appears.

FIG. 55 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.55, in which parts corresponding to those in FIG. 54 are assigned thesame codes as in FIG. 54, in transmitting system 3801 of OFDM-CDMAcommunication apparatus 3800 according to this embodiment, a knownsignal code division signal is shifted by one or more chips byserial/parallel converter (S/P) 3802 and parallel/serial converter (P/S)3803, and this is supplied via switch 3804 to adder 3805 only at thestart of a frame. By this means, a signal is obtained in which a codedivision signal shifted by one or more chips and the original codedivision signal are multiplexed only at the start of a frame.

Embodiment 39

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 39 of the present invention is that, in addition tomultiplexing a code-divided signal and that code-divided signal shiftedby one or more chips, and transmitting this code division multiplexedsignal assigned to a plurality of subcarriers, the number of chips bywhich the signal is shifted is made variable. By this means, it ispossible to prevent burst errors in addition to achieving the effect ofEmbodiment 34.

Here, a case will be considered in which processing that multiplexes acode division signal shifted by one or more chips is performed only fora specific transmit signal (hereinafter referred to as “user”), asdescribed in Embodiment 35. In a multipath environment, there are usersfor which disruption of orthogonality between spreading codes is severe.There are also users for which severe disruption of orthogonalitybetween spreading codes continues for a long period when the fluctuationspeed of the propagation environment is slow, and errors may occurcontinuously (generally referred to “burst errors”) for such users.

In consideration of this point, in this embodiment the number of chipsby which a signal is shifted is changed for every symbol transmitted,for example. By this means, it is possible to prevent the occurrence ofusers for which severe disruption of orthogonality between spreadingcodes continues for a long period. As a result, the possibility of bursterrors occurring can be greatly reduced.

FIG. 56 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.56, in which parts corresponding to those in FIG. 52 are assigned thesame codes as in FIG. 52, in transmitting system 3901 of OFDM-CDMAcommunication apparatus 3900, a transmit signal n code division signalis shifted by one or more chips by serial/parallel converter (S/P) 3902and parallel/serial converter (P/S) 3903, and the pre-shift codedivision signal and post-shift code division signal are multiplexed byadder 3904.

In addition, transmitting system 3901 is provided with a counter 3905that counts the number of symbols of transmit signal n, andparallel/serial converter 3903 shifts the spread signal in accordancewith the count value of counter 3905 by performing signal rearrangementin accordance with that count value. If, for example, a counter is usedthat counts up to four symbols, and is reset when the number of symbolsreaches 5, four kinds of shift amounts can be set.

According to the above configuration, by making the number of chips bywhich a signal is shifted variable, in addition to multiplexing acode-divided signal and that code-divided signal shifted by one or morechips, and transmitting this code division multiplexed signal assignedto a plurality of subcarriers, it is possible to reduce the probabilityof burst errors occurring, in addition to enabling error ratecharacteristics to be improved.

In this embodiment, a case has been described in which processing thatmultiplexes a code-divided signal, and that code-divided signal shiftedby one or more chips according to a symbol, and transmits this codedivision multiplexed signal assigned to a plurality of subcarriers, isperformed only for a specific transmit signal n, but the presentinvention is not limited to this, and the same kind of processing mayalso be performed, for example, for a known signal or for a plurality ofsignals. Also, the period for changing the shift amount is not limitedto once every symbol, and the period for changing the shift amount maybe selected according to the Doppler frequency, for example. Moreover,various methods may be applied, such as changing the shift amount atpredetermined intervals, for example.

Embodiment 40

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 40 of the present invention is that the degree ofmultiplexing of a signal obtained by shifting a code-divided signal byone or more chips, to be multiplexed with the code-divided signal, ismade variable. By this means, it is possible to effectively improve theerror rate characteristics of a specific user, while suppressing adecline of the error rate characteristics of other users.

The higher the degree of multiplexing of a signal obtained by shifting acode-divided signal by one or more chips, to be multiplexed with thecode-divided signal, the greater is the effect of quality improvementfor a poor-quality user (a user for which it is difficult to obtain thenecessary quality unless multiplexing is performed (for example, a userthat receives transmit signal n in FIG. 57)).

However, for other users (users for which the necessary quality can beadequately obtained without multiplexing (for example, users thatreceive transmit signals 4 k+1 . . . n−1)), it is better not to increasethe degree of multiplexing more than necessary, since the interferencecomponent increases accordingly.

Focusing on this point, this embodiment enables the error ratecharacteristics of a specific user to be effectively improved, whilesuppressing a decline of the error rate characteristics of other users,by selecting an appropriate degree of multiplexing of a signal obtainedby shifting a code-divided signal by one or more chips, according to thecircumstances.

In this embodiment, the degree of multiplexing is changed in accordancewith quality information (such as RSSI (Received Signal StrengthIndicator)) for the poorest-quality user, for example.

FIG. 57 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.57, in which parts corresponding to those in FIG. 52 described inEmbodiment 35 are assigned the same codes as in FIG. 52, transmittingsystem 4001 of OFDM-CDMA communication apparatus 4000 is provided with aplurality of parallel/serial converters (P/S) 4003A through 4003C thatshift by different numbers of chips as latter-stage sections withrespect to serial/parallel converter (S/P) 4002. For example,parallel/serial converter 4003A may form a code division signal shiftedby two chips, while parallel/serial converter 4003B forms a codedivision signal shifted by four chips, and parallel/serial converter4003C forms a code division signal shifted by six chips.

The code division signals of different shift amounts formed byparallel/serial converters 4003A through 4003C are input to selectingsection 4004. Also input to selecting section 4004 is a comparisonresult obtained by size comparison section 4006, which compares qualityinformation (such as RSSI) for the user to which transmit signal n istransmitted with a predetermined threshold value.

Based on the result of this comparison, selecting section 4004 selectsand outputs more code division signals the poorer the channel quality.For example, if the channel quality is very good, nothing is output; ifthe channel quality is rather poor, only input from parallel/serialconverter 4003A is output; and if the channel quality is very poor,inputs from all of parallel/serial converters 4003A through 4003C areoutput.

By this means, a code division multiplexed signal is obtained from adder4005 in which the degree of multiplexing of a signal shifted by one ormore chips differs according to the channel quality. This code divisionmultiplexed signal is then further code division multiplexed with codedivision signals for other users by adder B5.

According to the above configuration, by making the degree ofmultiplexing of a signal obtained by shifting a code-divided signal byone or more chips, to be multiplexed with the code-divided signal,variable, it is possible to effectively improve the error ratecharacteristics of a specific user, while suppressing a decline of theerror rate characteristics of other users.

Embodiment 41

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 41 of the present invention is that the degree ofmultiplexing of a signal obtained by shifting a code-divided signal byone or more chips, to be multiplexed with the code-divided signal, ischanged adaptively according to the degree of code multiplexing of othersignals code division multiplexed with this signal. By this means, it ispossible to effectively improve the error rate characteristics of aspecific user, while suppressing a decline of the error ratecharacteristics of other users.

As explained in Embodiment 35, a spreading code and a signal with thatspreading code shifted are not necessarily in an orthogonal relationship(having a mutual correlation of 0). Therefore, as the degree of signalmultiplexing increases, when code division signals shifted by one ormore chips are multiplexed, it may happen that the interferencecomponent increases and, conversely, error rate characteristics degrade.

In consideration of this point, in this embodiment the degree ofmultiplexing of a signal obtained by shifting a code-divided signal byone or more chips, to be multiplexed with the code-divided signal, ischanged adaptively according to the degree of code multiplexing (thenumber of transmit signals 4 k+1 through n code division multiplexed byadder B5 in FIG. 58). Specifically, when the degree of code multiplexingis low, the degree of multiplexing is made higher, and when the degreeof code multiplexing is high, the degree of multiplexing is made lower.By this means, it is possible to effectively improve the error ratecharacteristics of transmit signal n to a specific user, whilesuppressing a decline of the error rate characteristics of transmitsignals 4 k+1 . . . n−1 to other users.

FIG. 58 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.58, in which parts corresponding to those in FIG. 57 are assigned thesame codes as in FIG. 57, transmitting system 4101 of OFDM-CDMAcommunication apparatus 4100 is provided with a size comparison section4102, which compares information indicating the degree of multiplexing(the number of transmit signals 4 k+1 through n code divisionmultiplexed by adder B5) with a predetermined threshold value.

Based on the result of this comparison, selecting section 4103 selectsand outputs more code division signals the lower the degree ofmultiplexing. For example, if the degree of multiplexing is high, onlyinput from parallel/serial converter 4003A is output, and if the degreeof multiplexing is low, inputs from all of parallel/serial converters4003A through 4003C are output. By this means, a code divisionmultiplexed signal is obtained from adder 4005 in which the degree ofmultiplexing of a signal shifted by one or more chips differs accordingto the degree of multiplexing performed by adder B5.

According to the above configuration, by adaptively changing the degreeof multiplexing of a signal obtained by shifting a code-divided signalby one or more chips, to be multiplexed with the code-divided signal,according to the degree of code multiplexing of other signals (transmitsignals 4 k+1 . . . n−1) code division multiplexed with this signal(transmit signal n), it is possible to effectively improve the errorrate characteristics of a specific user, while suppressing a decline ofthe error rate characteristics of other users.

Embodiment 42

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 42 of the present invention is that, when multiplexing acode-divided signal with a signal obtained by shifting that code-dividedsignal by one or more chips, the number of chips by which the signal isshifted is changed according to the cell of the communicating party. Bythis means, it is possible for the communicating station to identify thecell to which the station itself belongs by detecting the number ofchips by which a signal is shifted. As a result, the number of cellsthat can be identified can be significantly increased.

When the number of chips by which a signal is shifted is changed, thetiming at which a correlation peak appears during reception (thecorrelator coefficient chip shift number) differs on the receiving side.Therefore, if the shifted number of chips is changed on the transmittingside according to the cell, cell identification can be performed on thereceiving side using the correlation peak timing. By this means, thenumber of cells that can be identified can be increased.

FIG. 59 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.59, in which parts corresponding to those in FIG. 52 are assigned thesame codes as in FIG. 52, in transmitting system 4201 of OFDM-CDMAcommunication apparatus 4200, a code division signal is shifted by anumber of chips in accordance with information indicating cellidentification from a control section (not shown) by means ofserial/parallel converter (S/P) 4202 and parallel/serial converter (P/S)4203, and the pre-shift code division signal and post-shift codedivision signal are multiplexed by adder 4204.

By this means, the far-end station that receives transmit signal ndetects the correlation peak timing when the received signal is despreadusing the same spreading code as spreading section An, and can performcell identification based on this timing.

According to the above configuration, by changing the number of chipsshifted according to the cell of the communicating party whenmultiplexing a code-divided signal with a signal obtained by shiftingthat code-divided signal by one or more chips, the number of cells thatcan be identified can be significantly increased.

Embodiment 43

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 43 of the present invention is that, whereas inabove-described Embodiment 38 a code division signal shifted by one ormore chips and the original code division signal are multiplexed only atthe start of a frame, in this embodiment a code division signal shiftedby one or more chips and the original code division signal aremultiplexed throughout an entire frame, and the number of chips shiftedis changed only at the start of a frame. By this means, it is possibleto significantly increase error rate characteristics in addition toachieving the effect of Embodiment 38.

In Embodiment 38, a code division signal shifted by one or more chipsand the original code division signal are multiplexed only at the startof a frame, enabling frame synchronization detection processing to beperformed on the receiving side without increasing the number ofspreading codes or increasing the number of known signals. However, aneffect of improvement in error rate characteristics due to multiplexingof a code division signal shifted by one or more chips with the originalcode division signal cannot be expected.

Thus, in this embodiment, a code division signal shifted by one or morechips and the original code division signal are multiplexed throughoutan entire frame, and the number of chips shifted is changed only at thestart of a frame, thereby enabling frame synchronization detection to beperformed, and also enabling an effect of improvement in error ratecharacteristics to be obtained.

FIG. 60 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.60, in which parts corresponding to those in FIG. 59 described inEmbodiment 42 are assigned the same codes as in FIG. 59, in transmittingsystem 4301 of OFDM-CDMA communication apparatus 4300, a code divisionsignal is shifted adaptively in accordance with information indicatingthe start of a frame from a control section (not shown) by means ofserial/parallel converter (S/P) 4302 and parallel/serial converter (P/S)4303. Specifically, the number of chips shifted at the start of a frameand the number of chips shifted other than at the start of a frame aremade to differ. The code division signal shifted in this way and theoriginal code division signal are multiplexed by adder 4304.

According to the above configuration, by multiplexing a code divisionsignal shifted by one or more chips and the original code divisionsignal throughout an entire frame, and changing the number of chipsshifted only at the start of a frame, it is possible to perform framesynchronization detection, and also to obtain an effect of improvementin error rate characteristics.

Embodiment 44

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 44 of the present invention is that the interval forinsertion of a propagation path estimation preamble is set independentlyfor each subcarrier. By this means, it is possible to improve the errorrate characteristics of a user with large propagation path fluctuations,such as a user moving at high speed, with almost no lowering oftransmission efficiency.

As speed of movement increases, the speed of propagation pathfluctuations also increases, and therefore degradation of error ratecharacteristics becomes greater. There is a method whereby thepropagation path estimation preamble insertion period is shortened inorder to prevent degradation of the error rate characteristics of a usermoving at high speed. However, as a propagation path estimation preambleis not data, a problem arises of transmission efficiency falling inproportion as the number of insertions increases.

In consideration of this point, in this embodiment the insertioninterval for a propagation path estimation preamble is shortened forsubcarriers for which the degree of signal multiplexing is set low, asshown in FIG. 61, and these subcarriers are assigned to a user moving athigh speed. As the propagation path estimation preamble insertioninterval is not shortened for subcarriers other than those for which thedegree of signal multiplexing is set low, a fall in transmissionefficiency can be prevented. By this means, it is possible to improvethe error rate characteristics of a user moving at high speed, withalmost no lowering of transmission efficiency.

FIG. 62 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.62, in which parts corresponding to those in FIG. 5 described inEmbodiment 1 are assigned the same codes as in FIG. 5, transmittingsystem 4401 of OFDM-CDMA communication apparatus 4400 according to thisembodiment is provided with a parallel/serial converter (P/S) 4402 as asection for arranging propagation path estimation preambles for signalsof subcarriers for which the degree of signal multiplexing is set high,and a parallel/serial converter (P/S) 4403 as a section for arrangingpropagation path estimation preambles for signals of subcarriers forwhich the degree of signal multiplexing is set low.

Parallel/serial converter (P/S) 4402 does not shorten the propagationpath estimation preamble insertion interval, while parallel/serialconverter (P/S) 4403 does shorten the propagation path estimation signalinsertion interval—that is to say, they respectively arrange propagationpath estimation preambles as shown in FIG. 61—and the signals for whichpropagation path estimation preamble arrangement has been performed aresent to next-stage parallel/serial converter (P/S) 102.

According to the above configuration, by setting the interval forinsertion of a propagation path estimation preamble independently foreach subcarrier, it is possible to improve the error ratecharacteristics of specific transmit signals, with almost no lowering oftransmission efficiency.

In this embodiment, the error rate characteristics of signalstransmitted by subcarriers for which the degree of signal multiplexingis set low are improved by shortening the propagation path estimationpreamble insertion interval of subcarriers for which the degree ofsignal multiplexing is set low, but it is also possible for subcarrierswhose propagation path estimation preamble insertion interval isshortened not to be limited to subcarriers for which the degree ofsignal multiplexing is set low, but to be changed as appropriateaccording to propagation path fluctuation conditions.

Embodiment 45

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 45 of the present invention is that a differential modulationmethod is used as the modulation method for subcarriers for which thedegree of signal multiplexing is set low. By this means, it is possibleto improve the error rate characteristics of a user moving at highspeed.

A modulation method that is tolerant of high-speed propagation pathfluctuations is differentially coherent detection. In differentiallycoherent detection, the phase difference from data of the precedingsymbol is taken as the modulation result, and therefore only channelfluctuation effects for one symbol are received. In order to performdifferentially coherent detection, a differential modulation method suchas DQPSK must be used as the modulation method, and thereforedifferentially coherent detection cannot be applied to an M-arymodulation method such as 16QAM. There is thus a problem of a fall intransmission efficiency if differentially coherent detection isperformed for all subcarriers.

However, if differential modulation is performed only for users movingat high speed, the error rate characteristics of users moving at highspeed can be improved with almost no lowering of transmissionefficiency.

FIG. 63 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.63, in which parts corresponding to those in FIG. 6 are assigned thesame codes as in FIG. 6, transmitting system 4501 of OFDM-CDMAcommunication apparatus 4500 is provided with modulation circuits F1through F(4 k) and F(4 k+1) through Gn, which perform modulationprocessing on transmit signals 1 through n.

Modulation circuits F(4 k+1) through Gn corresponding to transmitsignals 4 k+1 through n with a low degree of signal multiplexing performdifferential modulation (such as D8PSK or DQPSK), while modulationcircuits F1 through F(4 k) corresponding to transmit signals with a highdegree of signal multiplexing perform modulation other than differentialmodulation, such as 16QAM.

In receiving system 4510, a signal that has undergone FFT processing isdivided into a signal with a low degree of multiplexing and a signalwith a high degree of multiplexing by serial/parallel converter (S/P)4511, and the signal with a high degree of multiplexing is restored viapropagation path compensation section 4512 and despreader 4513 and takenas a received signal. Meanwhile, the signal with a low degree ofmultiplexing is subjected to differentially coherent detection bydifferentially coherent detection section 4514, and is then restored bydespreader 4515 and taken as a received signal. A description ofdifferentially coherent detection itself is omitted here, as it iswell-known technology.

Thus, by performing differential modulation only on a transmit signalassigned to subcarriers with a low degree of signal multiplexing, andexecuting differentially coherent detection processing on the receivingside, it is possible to improve the error rate characteristics of a usermoving at high speed, with almost no lowering of transmissionefficiency.

In this embodiment, a case has been illustrated in which differentialmodulation is used on a fixed basis for subcarriers for which the degreeof multiplexing is set low, but the present invention is not limited tothis, and it is also possible for the modulation method of subcarriersfor which the degree of multiplexing is set low to be switchedadaptively between differential modulation and another modulationmethod. For example, it is possible for the modulation method ofsubcarriers for which the degree of multiplexing is set low to beswitched adaptively between differential modulation and anothermodulation method according to the speed of movement (for example, it isalso possible to use the difference between the present channelestimation result and the previous channel estimation result).

Embodiment 46

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 46 of the present invention is that, when transmission isperformed using OFDM-CDMA, subcarriers to which spread signals areallocated only in the frequency axis direction, and subcarriers to whichspread signals are allocated in both the frequency axis direction andthe time axis direction, are formed.

In this embodiment, the degree of signal multiplexing is setindividually for each subcarrier. Also, as shown in FIG. 5, the degreeof signal multiplexing is set low for one subcarrier group G1 (indicatedby shading in FIG. 5) of the five subcarrier groups.

In addition, in this embodiment, chips that have undergone spreadingprocessing are allocated in both the frequency axis direction and thetime axis direction for subcarrier group G1. On the other hand, chipsthat have undergone spreading processing are allocated only in thefrequency axis direction for other subcarrier groups.

As a result, it is possible to significantly improve error ratecharacteristics for subcarrier group G1, with no decrease in spectralefficiency whatever for subcarrier groups other than subcarrier group G1among the five subcarrier groups. By this means, it is possible to makespectral efficiency compatible with error rate characteristics.

In this embodiment, a signal for a communicating party whose channelquality is poor, such as a distant radio station or a radio station witha poor SIR (Signal to Interference Ratio), for example, is allocated tosubcarrier group G1. By this means, it is possible to improve the errorrate characteristics of a communicating party with poor channel quality,with almost no lowering of spectral efficiency.

As chips that have undergone spreading processing are allocated in boththe frequency axis direction and the time axis direction for subcarriergroup G1, even if the spreading ratio of a spread signal allocated tothis subcarrier group G1 is made larger than the spreading ratio ofspread signals allocated to other groups, it is possible to prevent thefrequency band of other subcarrier groups from decreasing.

In this embodiment, the spreading ratio of subcarrier groups other thansubcarrier group G1 is made 1/5, and the spreading ratio of subcarriergroup G1 is set to twice that of other subcarrier groups. However,spreading ratios are not limited to these, and a spreading ratio can beset individually for each subcarrier group.

An actual sample configuration of an OFDM-CDMA communication apparatusaccording to this embodiment will now be described, using FIG. 64.Transmitting system 4601 of OFDM-CDMA communication apparatus 4600divides n transmit signals 1 through n into 5 groups, and forms anOFDM-CDMA signal by spreading transmit signals in each group in thefrequency axis direction only of a plurality of subcarriers, or in boththe frequency axis direction and the time axis direction (so-called“two-dimensional spreading”).

In actuality, in transmitting system 4601, transmit signals 1 through nare input to spreaders A1 through An, which perform spreading processingusing different spreading codes. The spreading ratio of spreaders A(4k+1) through An is set to a higher value than the spreading ratio of theother spreaders A1 through A(4 k) For example, the spreading ratio ofspreaders A(4 k+1) through An may be set to 1/5 the number ofsubcarriers while the spreading ratio of spreaders A1 through A(4 k) isset to twice that value.

Spread signals are multiplexed by adders B1 through B5, one of which isprovided for each group (in this embodiment, each of 5 groups), and bythis means, code division multiplexed signals S1 through S5corresponding to the predetermined number of groups are obtained.

Here, in transmitting system 4601, adders B1 through B4 form codedivision multiplexed signals S1 through S4 in each of which k transmitsignals are multiplexed, whereas adder B5 forms code divisionmultiplexed signal S5 in which fewer than k transmit signals aremultiplexed. That is to say, the number of signals (n−4 k) of transmitsignals (4 k+1) through n that are code division multiplexed by adder B5is selected so that 1<(n−4 k)<k.

Code division multiplexed signals S1 through S5 obtained by adders B1through B5, respectively, are input to a parallel/serial converter (P/S)4602 as a spread signal assigning section. Parallel/serial converter4602, comprising memory and a flip-flop circuit, rearranges codedivision multiplexed signals S1 through S5 in a predetermined order, andoutputs them as a serial signal S6. In this embodiment, this arrangementorder determines which subcarrier groups in FIG. 5 code divisionmultiplexed signals S1 through S5 are allocated to, and also determineswhether allocation is performed only in the frequency axis direction orin both the frequency axis direction and the time axis direction.

In this embodiment, code division multiplexed signal S5 for which thedegree of signal multiplexing is set low and the spreading ratio is sethigh is allocated spread in both the frequency axis direction and thetime axis direction, and the other code division multiplexed signals S1through S4 are allocated spread only in the frequency axis direction.

Serial signal S6 output from parallel/serial converter 4602 is input toinverse fast Fourier transform circuit (IFFT) 103. Inverse fast Fouriertransform circuit 103 executes inverse fast Fourier transform processingon serial signal S6 for each of code division multiplexed signals S1through S5, and thereby allocates spread chips by distributing themamong a plurality of subcarriers that are in a mutually orthogonalrelationship.

At this time, code division multiplexed signal S1 that was code divisionmultiplexed by adder B1, for example, is allocated by frequency domainspreading to a certain subcarrier group, and code division multiplexedsignal S5 that was code division multiplexed by adder B5 is allocated byspreading in both the frequency axis direction and the time axisdirection to subcarrier group G1 in FIG. 5.

In this way, it is possible to form an OFDM-CDMA signal S7 in which aspread signal spread in both the frequency axis direction and the timeaxis direction is allocated to subcarrier group G1, and spread signalsspread only in the frequency axis direction are allocated to othersubcarrier groups. Obtained OFDM-CDMA signal S7 is then transmitted viaradio transmitting section (RF) 104 that performs radio transmissionprocessing such as digital/analog conversion and signal amplification,and antenna AN.

FIG. 65 shows an example of OFDM-CDMA signal S7 formed by OFDM-CDMAcommunication apparatus 4600. As can be seen from this drawing, codedivision multiplexed signal S5 obtained by spreading with a spreadingratio twice that of other code division multiplexed signals S1 throughS4, is allocated by two-dimensional spreading to subcarriers in both thefrequency axis direction and the time axis direction (that is, thespread signal is allocated over a plurality of symbols).

In the above configuration, by providing a subcarrier group G1 in whichspread signals are allocated in both the frequency axis direction andthe time axis direction, it is possible to prevent the frequency band ofother subcarrier groups from being decreased even if the spreading ratioof a spread signal allocated to this subcarrier group G1 is increased.

Also, by providing subcarrier groups in which spread signals areallocated only in the frequency axis direction, wasteful band usage dueto subcarriers that do not transmit any data can be prevented ascompared with the case where spread signals are allocated in both thefrequency axis direction and the time axis direction for allsubcarriers.

Moreover, by making the degree of multiplexing of transmit signals 4 k+1through n of subcarrier group G1, in which spread signals are allocatedin both the frequency axis direction and the time axis direction, lowerthan the degree of signal multiplexing of other subcarrier groups,inter-code interference on the propagation path is reduced, andtherefore the error rate characteristics of signals superimposed onsubcarriers in which spread signals are allocated in both the frequencyaxis direction and the time axis direction can be significantlyimproved.

As a result, compared with a case in which the degree of multiplexingand spreading direction are decided uniformly for all subcarriers (forexample, when it is decided that spread signals are to be allocated inboth the frequency axis direction and the time axis direction for allsubcarriers) if transmit signals (4 k+1) through n for which it isdesired to improve error rate characteristics, such as importantinformation, are spread in both the frequency axis direction and thetime axis direction and also allocated to subcarriers with a low degreeof signal multiplexing, and transmit signals 1 through k, . . . , (3k+1) through 4 k for which the error rate need not be improved so muchare spread only in the frequency axis direction and also allocated tosubcarriers with a high degree of signal multiplexing, degradation oferror rate characteristics can be prevented without lowering spectralefficiency significantly.

Thus, by selecting independently for each subcarrier whether spreadsignals are to be allocated only in the frequency axis direction, or areto be allocated in both the frequency axis direction and the time axisdirection, an OFDM-CDMA communication apparatus 4600 can be implementedthat enables spectral efficiency and error rate characteristics to bemade compatible.

Furthermore, by making the degree of signal multiplexing of subcarriersfor which spread signals are allocated in both the frequency axisdirection and the time axis direction higher than the degree of signalmultiplexing of other subcarriers, it is possible to significantlyimprove the error rate characteristics of signals superimposed on thesesubcarriers.

In this embodiment, a case has been described in which a code divisionmultiplexed signal for which the degree of signal multiplexing is setlower than for others is spread in both the frequency axis direction andthe time axis direction, but the present invention is not limited tothis, and it is possible to select independently for any subcarrierwhether a signal spread only in the frequency axis direction is to beallocated, or a signal spread in both the frequency axis direction andthe time axis direction is to be allocated.

Also, in FIG. 65 of this embodiment a case has been illustrated in whicha signal spread over two symbols is allocated as regards the time axisdirection, but the present invention is not limited to this, andallocation is also possible over any number of symbols.

Embodiment 47

A special feature of an OFDM-CDMA communication apparatus according toEmbodiment 47 of the present invention is that, as shown in FIG. 66, forsubcarriers in which spread signals are allocated in both the frequencyaxis direction and the time axis direction, the degree of signalmultiplexing is further decreased, and spread signals are allocated onlyin the frequency axis direction at intervals of a plurality of symbols.

By this means, the receiving side need only perform processing from FFT(fast Fourier transform) onward for symbols to which a signal isallocated, enabling power consumption to be further reduced comparedwith Embodiment 1.

FIG. 67 shows the configuration of the transmitting system of anOFDM-CDMA communication apparatus according to this embodiment. In FIG.67, in which parts corresponding to those in FIG. 64 are assigned thesame codes as in FIG. 64, in transmitting system 4701 of OFDM-CDMAcommunication apparatus 4700, users allocated to subcarriers for whichspread signals are assigned in both the frequency axis direction and thetime axis direction (transmit signals 4 k+1 through n) are divided intotwo groups, for example.

Specifically, a code division multiplexed signal S5-1 is obtained bymultiplexing spread signals of half of the users by means of adder B5 a,and a code division multiplexed signal S5-2 is obtained by multiplexingspread signals of the other half of the users by means of adder B5 b.These code division multiplexed signals S5-1 and S5-2 are then sent toparallel/serial converter (P/S) 102 in a predetermined order viaparallel/serial converter (P/S) 4702.

By this means, in transmitting system 4701, as shown in FIG. 66, half ofthe users are allocated to the first symbol (transmit signals 4 k+1through n/2) and the other half of the users are allocated to the secondsymbol (transmit signals n/2+1 through n) (that is, they are allocatedin the time axis direction). Looking at subcarriers of the same time,code division multiplexed signals of half of the users are allocated inthe frequency axis direction. Thereafter, spread signals continue to beallocated to alternate symbols in the same way.

According to the above configuration, when allocating spread signals inboth the frequency axis direction and the time axis direction, aplurality of transmit signals 1 through n are divided into at least twogroups, a code division multiplexed signal of first group 4 k+1 throughn/2 and a code division multiplexed signal of second group n/2+1 throughn are allocated alternately in the time axis direction, and a first orsecond group code division multiplexed signal is allocated in thefrequency axis direction within the same time, so that on the receivingside, it is only necessary to process code division multiplexed signals(symbols) of the same group in the same time—that is, to performprocessing from FFT (fast Fourier transform) onward only for symbolsallocated to subcarriers of the same time—thereby enabling powerconsumption to be further reduced.

In this embodiment, a case has been described in which spread signalsare allocated every other symbol, but the present invention is notlimited to this, and spread symbols can be allocated at intervals of anynumber of symbols.

The present invention is not limited to the above-described embodiments,and various variations and modifications may be possible withoutdeparting from the scope of the present invention.

A radio transmitting apparatus according to the present invention has aconfiguration comprising a plurality of spreading sections that spread aplurality of transmit signals using different spreading codes, and aspread signal assigning section that assigns a spread signal obtained bya spreading section to a plurality of subcarriers, wherein the spreadsignal assigning section assigns a spread signal only in the frequencyaxis direction, or in both the frequency axis direction and the timeaxis direction, independently for each subcarrier.

According to this configuration, by providing subcarriers to whichspread signals are allocated in both the frequency axis direction andthe time axis direction., it is possible to prevent the frequency bandof other subcarriers from decreasing even if the spreading ratio ofspread signals allocated to those subcarriers is increased. Also, byproviding subcarriers to which spread signals are allocated only in thefrequency axis direction, wasteful band usage due to subcarriers that donot transmit any data can be prevented as compared with the case wherespread signals are allocated in both the frequency axis direction andthe time axis direction for all subcarriers. As a result, spectralefficiency and error rate characteristics can be made compatible.

A radio transmitting apparatus according to the present invention has aconfiguration wherein the spreading ratios of the above-describedplurality of spreading sections are set to different values.

A radio transmitting apparatus according to the present invention has aconfiguration further comprising a first multiplexing section thatmultiplexes a first number of spread signals among the plurality ofspread signals obtained by the above-described spreading sections, and asecond multiplexing section that multiplexes a second number of spreadsignals smaller than the first number among the spread signals notmultiplexed by that first multiplexing section, wherein theabove-described spread signal assigning section assigns a code divisionmultiplexed signal obtained by the first multiplexing section tofrequency axis direction subcarriers, and assigns a code divisionmultiplexed signal obtained by the second multiplexing section to bothfrequency axis direction and time axis direction subcarriers.

According to this configuration, inter-code interference on thepropagation path is lower for subcarriers for which the degree oftransmit signal multiplexing is low than for subcarriers for which thedegree of transmit signal multiplexing is high, and it is thereforepossible to significantly improve the error rate characteristics ofsignals superimposed on subcarriers in which spread signals areallocated in both the frequency axis direction and the time axisdirection.

A radio transmitting apparatus according to the present invention has aconfiguration wherein, of the above-described spreading sections, thespreading ratio of a spreading section that forms spread signalsassigned in both the frequency axis direction and the time axisdirection is set to a value greater than the spreading ratio of aspreading section that forms spread signals assigned only in thefrequency axis direction.

According to this configuration, since the spreading ratio ofsubcarriers in which spread signals are allocated in both the frequencyaxis direction and the time axis direction is increased, thesignal-to-noise ratio of spread signals superimposed on thesesubcarriers increases, and error rate characteristics improve. Also, assignals are allocated in both the frequency axis direction and the timeaxis direction, even if the spreading ratio is increased, spread signalscan be accommodated within the stipulated subcarrier frequency band, andthe frequency band of other subcarriers need not be narrowed.

A radio transmitting apparatus according to the present invention has aconfiguration further comprising first and second multiplexing sectionsthat multiplex respective predetermined numbers of spread signals amongthe plurality of spread signals obtained by the above-describedspreading sections, wherein the spread signal assigning section, whenassigning in both the frequency axis direction and the time axisdirection, allocates signals multiplexed by the first and secondmultiplexing sections alternately in the time axis direction, and,within the same time, allocates a signal multiplexed by the first orsecond multiplexing section in the frequency axis direction.

According to this configuration, on the receiving side that receivesthis OFDM-CDMA signal, it is only necessary to process code divisionmultiplexed signals (symbols) of the same group in the same time—thatis, to perform processing from FFT (fast Fourier transform) onward onlyfor symbols allocated to subcarriers of the same time—thereby enablingpower consumption to be reduced.

As described above, according to the present invention an OFDM-CDMAradio transmitting apparatus and radio receiving apparatus areimplemented that make it possible to make spectral efficiency compatiblewith error rate characteristics, by forming subcarriers to which spreadsignals are allocated only in the frequency axis direction, andsubcarriers to which spread signals are allocated in both the frequencyaxis direction and the time axis direction, when performing radiotransmission using the OFDM-CDMA method.

This application is based on Japanese Patent Application No. 2001-359964filed on Nov. 26, 2001, Japanese Patent Application No. 2002-31243 filedon Feb. 7, 2002, and Japanese Patent Application No. 2002-115537 filedon Apr. 17, 2002, entire contents of which are expressly incorporated byreference herein.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a radio communication system thatuses the OFDM-CDMA method.

1. A radio transmitting apparatus comprising: a plurality of spreadingsections that spread a plurality of transmit signals using spreadingcodes different from each other; an orthogonal frequency divisionmultiplexing section that performs orthogonal frequency divisionmultiplexing on spread signals obtained by the spreading sections; and aspread signal assigning section that selects between assigning thespread signals only in a frequency axis direction and assigning thespread signals in both the frequency axis direction and time axisdirection, on a per subcarrier group basis, based on channel quality ofeach subcarrier or an error rate required for the transmit signals,wherein: the spread signal assigning section assigns the spread signalsso that a subcarrier group in which the spread signals are assigned onlyin the frequency axis direction and a subcarrier group in which thespread signals are assigned in both the frequency axis direction and thetime axis direction are present in a same time.
 2. A radio transmittingapparatus comprising: a plurality of spreading sections that spread aplurality of transmit signals using spreading codes different from eachother; an orthogonal frequency division multiplexing section thatperforms orthogonal frequency division multiplexing on spread signalsobtained by the spreading sections; a spread signal assigning sectionthat selects between assigning the spread signals only in a frequencyaxis direction and assigning the spread signals in both the frequencyaxis direction and time axis direction, on a per subcarrier group basis,based on channel quality of each subcarrier or an error rate requiredfor the transmit signals; a first multiplexing section that multiplexesa first number of spread signals among a plurality of spread signalsobtained by the spreading sections; and a second multiplexing sectionthat multiplexes a second number of spread signals smaller than thefirst number among spread signals not multiplexed by the firstmultiplexing section, wherein: the spread signal assigning sectionassigns a code division multiplexed signal obtained by the firstmultiplexing section only in the frequency axis direction, and assigns acode division multiplexed signal obtained by the second multiplexingsection in both the frequency axis direction and the time axisdirection.
 3. A radio transmitting apparatus comprising: a plurality ofspreading sections that spread a plurality of transmit signals usingspreading codes different from each other; an orthogonal frequencydivision multiplexing section that performs orthogonal frequencydivision multiplexing on spread signals obtained by the spreadingsections; and a spread signal assigning section that selects betweenassigning the spread signals only in a frequency axis direction andassigning the spread signals in both the frequency axis direction andtime axis direction, on a per subcarrier group basis, based on channelquality of each subcarrier or an error rate required for the transmitsignals, wherein: among the spreading sections, a spreading ratio of thespreading section that forms spread signals assigned in both thefrequency axis direction and the time axis direction is set to a valuegreater than a spreading ratio of the spreading section that formsspread signals assigned only in the frequency axis direction.
 4. A radiotransmitting apparatus comprising: a plurality of spreading sectionsthat spread a plurality of transmit signals using spreading codesdifferent from each other; an orthogonal frequency division multiplexingsection that performs orthogonal frequency division multiplexing onspread signals obtained by the spreading sections; a spread signalassigning section that selects between assigning the spread signals onlyin a frequency axis direction and assigning the spread signals in boththe frequency axis direction and time axis direction, on a persubcarrier group basis, based on channel quality of each subcarrier oran error rate required for the transmit signals; and first and secondmultiplexing sections that multiplex respective predetermined numbers ofspread signals among a plurality of spread signals obtained by thespreading sections, wherein: the spread signal assigning section, whenassigning in both the frequency axis direction and the time axisdirection, assigns signals multiplexed by the first multiplexing sectionand signals multiplexed by the second multiplexing section alternatelyin the time axis direction, and within a same time, assigns signalsmultiplexed by the first multiplexing section or signals multiplexed bythe second multiplexing section only in the frequency axis direction.